U.S. patent number 8,513,054 [Application Number 13/767,335] was granted by the patent office on 2013-08-20 for method for manufacturing semiconductor device.
This patent grant is currently assigned to Semiconductor Energy Laboratory Co., Ltd.. The grantee listed for this patent is Semiconductor Energy Laboratory Co., Ltd.. Invention is credited to Hiroki Ohara, Junichiro Sakata, Toshinari Sasaki, Shunpei Yamazaki.
United States Patent |
8,513,054 |
Sasaki , et al. |
August 20, 2013 |
**Please see images for:
( Certificate of Correction ) ** |
Method for manufacturing semiconductor device
Abstract
It is an object to provide a highly reliable semiconductor
device which includes a thin film transistor having stable electric
characteristics. It is another object to manufacture a highly
reliable semiconductor device at lower cost with high productivity.
In a method for manufacturing a semiconductor device which includes
a thin film transistor where a semiconductor layer having a channel
formation region, a source region, and a drain region are formed
using an oxide semiconductor layer, heat treatment (heat treatment
for dehydration or dehydrogenation) is performed so as to improve
the purity of the oxide semiconductor layer and reduce impurities
such as moisture. Moreover, the oxide semiconductor layer subjected
to the heat treatment is slowly cooled under an oxygen
atmosphere.
Inventors: |
Sasaki; Toshinari (Atsugi,
JP), Sakata; Junichiro (Atsugi, JP), Ohara;
Hiroki (Sagamihara, JP), Yamazaki; Shunpei
(Setagaya, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Semiconductor Energy Laboratory Co., Ltd. |
Atsugi |
N/A |
JP |
|
|
Assignee: |
Semiconductor Energy Laboratory
Co., Ltd. (Atsugi-shi, Kanagawa-ken, JP)
|
Family
ID: |
43410955 |
Appl.
No.: |
13/767,335 |
Filed: |
February 14, 2013 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20130157411 A1 |
Jun 20, 2013 |
|
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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13495481 |
Jun 13, 2012 |
8394671 |
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12826015 |
Jul 10, 2012 |
8216878 |
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Foreign Application Priority Data
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Jun 30, 2009 [JP] |
|
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2009-156414 |
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Current U.S.
Class: |
438/104;
257/E21.459; 257/E21.411; 438/151 |
Current CPC
Class: |
H01L
29/66742 (20130101); H01L 29/78618 (20130101); H01L
29/78696 (20130101); H01L 29/24 (20130101); H01L
29/42356 (20130101); H01L 29/7869 (20130101); H01L
29/78606 (20130101); H01L 27/1225 (20130101); H01L
27/124 (20130101); H01L 29/42384 (20130101); H01L
29/66969 (20130101) |
Current International
Class: |
H01L
21/00 (20060101) |
Field of
Search: |
;257/E21.411,E21.459
;438/104,151 |
References Cited
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WO |
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WO 2008/143304 |
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Nov 2008 |
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WO |
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WO-2009/034953 |
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Mar 2009 |
|
WO |
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|
Primary Examiner: Maldonado; Julio J
Assistant Examiner: Bachner; Robert
Attorney, Agent or Firm: Robinson; Eric J. Robinson
Intellectual Property Law Office, P.C.
Claims
What is claimed is:
1. A method for manufacturing a semiconductor device comprising the
steps of: forming an insulating layer over a substrate, forming a
first oxide semiconductor layer over the insulating layer; forming
a second oxide semiconductor layer over the first oxide
semiconductor layer; performing a first heat treatment on the first
oxide semiconductor layer and the second oxide semiconductor layer;
after the first heat treatment, cooling the first oxide
semiconductor layer and the second oxide semiconductor layer in an
atmosphere containing oxygen; forming a conductive film over the
first oxide semiconductor layer and the second oxide semiconductor
layer after the first heat treatment; etching the conductive film
to form a source electrode layer and a drain electrode layer; and
forming an oxide insulating film in contact with part of the second
oxide semiconductor layer.
2. The method for manufacturing a semiconductor device according to
claim 1, wherein a condition for forming the first oxide
semiconductor layer and a condition for forming the second oxide
semiconductor layer are different from each other.
3. The method for manufacturing a semiconductor device according to
claim 1, wherein the first heat treatment is performed under
reduced pressure.
4. The method for manufacturing a semiconductor device according to
claim 1, wherein the first heat treatment is performed in an inert
atmosphere.
5. The method for manufacturing a semiconductor device according to
claim 1, wherein hydrogen concentrations in the first oxide
semiconductor layer and the second oxide semiconductor layer are
reduced by the first heat treatment.
6. The method for manufacturing a semiconductor device according to
claim 1, wherein the first heat treatment is performed by a gas
rapid thermal anneal method or a lamp thermal anneal method.
7. The method for manufacturing a semiconductor device according to
claim 1, further comprising a step of forming an electrode over the
oxide insulating film.
8. The method for manufacturing a semiconductor device according to
claim 1, further comprising a step of performing a second heat
treatment after the oxide insulating film is formed.
9. The method for manufacturing a semiconductor device according to
claim 1, wherein each of the first oxide semiconductor layer and
the second oxide semiconductor layer includes indium and zinc.
10. A method for manufacturing a semiconductor device comprising
the steps of: forming an insulating layer over a substrate, forming
a first oxide semiconductor layer over the insulating layer;
forming a second oxide semiconductor layer over the first oxide
semiconductor layer; performing a first heat treatment on the first
oxide semiconductor layer and the second oxide semiconductor layer;
after the first heat treatment, cooling the first oxide
semiconductor layer and the second oxide semiconductor layer in an
atmosphere containing oxygen; forming a conductive film over the
first oxide semiconductor layer and the second oxide semiconductor
layer after the first heat treatment; etching the conductive film
to form a source electrode layer and a drain electrode layer;
forming an oxide insulating film in contact with part of the second
oxide semiconductor layer; and performing a second heat treatment
after the oxide insulating film is formed, wherein the first heat
treatment is performed at a first temperature higher or equal to
400.degree. C.
11. The method for manufacturing a semiconductor device according
to claim 10, wherein a condition for forming the first oxide
semiconductor layer and a condition for forming the second oxide
semiconductor layer are different from each other.
12. The method for manufacturing a semiconductor device according
to claim 10, wherein the first heat treatment is performed under
reduced pressure.
13. The method for manufacturing a semiconductor device according
to claim 10, wherein the first heat treatment is performed in an
inert atmosphere.
14. The method for manufacturing a semiconductor device according
to claim 10, wherein the second heat treatment is performed at a
second temperature which is lower than the first temperature.
15. The method for manufacturing a semiconductor device according
to claim 10, wherein hydrogen concentrations in the first oxide
semiconductor layer and the second oxide semiconductor layer are
reduced by the first heat treatment.
16. The method for manufacturing a semiconductor device according
to claim 10, further comprising a step of forming an electrode over
the oxide insulating film.
17. The method for manufacturing a semiconductor device according
to claim 10, wherein each of the first oxide semiconductor layer
and the second oxide semiconductor layer includes indium and
zinc.
18. A method for manufacturing a semiconductor device comprising
the steps of: forming an insulating layer over a substrate, forming
a first oxide semiconductor layer over the insulating layer;
forming a second oxide semiconductor layer including crystals over
the first oxide semiconductor layer; performing a first heat
treatment on the first oxide semiconductor layer and the second
oxide semiconductor layer; after the first heat treatment, cooling
the first oxide semiconductor layer and the second oxide
semiconductor layer in an atmosphere containing oxygen; forming a
conductive film over the first oxide semiconductor layer and the
second oxide semiconductor layer after the first heat treatment;
etching the conductive film to form a source electrode layer and a
drain electrode layer; and forming an oxide insulating film in
contact with part of the second oxide semiconductor layer.
19. The method for manufacturing a semiconductor device according
to claim 18, wherein a condition for forming the first oxide
semiconductor layer and a condition for forming the second oxide
semiconductor layer are different from each other.
20. The method for manufacturing a semiconductor device according
to claim 18, wherein the first heat treatment is performed under
reduced pressure.
21. The method for manufacturing a semiconductor device according
to claim 18, wherein the first heat treatment is performed in an
inert atmosphere.
22. The method for manufacturing a semiconductor device according
to claim 18, wherein hydrogen concentrations in the first oxide
semiconductor layer and the second oxide semiconductor layer are
reduced by the first heat treatment.
23. The method for manufacturing a semiconductor device according
to claim 18, further comprising a step of forming an electrode over
the oxide insulating film.
24. The method for manufacturing a semiconductor device according
to claim 18, further comprising a step of performing a second heat
treatment after the oxide insulating film is formed.
25. The method for manufacturing a semiconductor device according
to claim 18, wherein the first oxide semiconductor layer includes
crystal after the first heat treatment is performed.
26. The method for manufacturing a semiconductor device according
to claim 18, wherein each of the first oxide semiconductor layer
and the second oxide semiconductor layer includes indium and
zinc.
27. A method for manufacturing a semiconductor device comprising
the steps of: forming an insulating layer over a substrate, forming
a first oxide semiconductor layer over the insulating layer;
forming a second oxide semiconductor layer over the first oxide
semiconductor layer; performing a first heat treatment on the first
oxide semiconductor layer and the second oxide semiconductor layer;
after the first heat treatment, supplying oxygen to the first oxide
semiconductor layer and the second oxide semiconductor layer in an
atmosphere containing oxygen; forming a conductive film over the
first oxide semiconductor layer and the second oxide semiconductor
layer after the first heat treatment; etching the conductive film
to form a source electrode layer and a drain electrode layer; and
forming an oxide insulating film in contact with part of the second
oxide semiconductor layer.
28. The method for manufacturing a semiconductor device according
to claim 27, wherein a condition for forming the first oxide
semiconductor layer and a condition for forming the second oxide
semiconductor layer are different from each other.
29. The method for manufacturing a semiconductor device according
to claim 27, wherein the first heat treatment is performed under
reduced pressure.
30. The method for manufacturing a semiconductor device according
to claim 27, wherein the first heat treatment is performed in an
inert atmosphere.
31. The method for manufacturing a semiconductor device according
to claim 27, wherein hydrogen concentrations in the first oxide
semiconductor layer and the second oxide semiconductor layer are
reduced by the first heat treatment.
32. The method for manufacturing a semiconductor device according
to claim 27, wherein the first heat treatment is performed by a gas
rapid thermal anneal method or a lamp thermal anneal method.
33. The method for manufacturing a semiconductor device according
to claim 27, further comprising a step of forming an electrode over
the oxide insulating film.
34. The method for manufacturing a semiconductor device according
to claim 27, further comprising a step of performing a second heat
treatment after the oxide insulating film is formed.
35. The method for manufacturing a semiconductor device according
to claim 27, wherein each of the first oxide semiconductor layer
and the second oxide semiconductor layer includes indium and zinc.
Description
TECHNICAL FIELD
The present invention relates to a method for manufacturing a
semiconductor device including an oxide semiconductor.
BACKGROUND ART
In recent years, a technique by which a thin film transistor (TFT)
is manufactured using a semiconductor thin film (having a thickness
of approximately several nanometers to several hundred nanometers)
formed over a substrate having an insulating surface has attracted
attention. Thin film transistors are applied to a wide range of
electronic devices such as ICs or electro-optical devices and
urgently developed particularly as switching elements in image
display devices.
Various metal oxides are used for a variety of applications. Indium
oxide is a well-known material and is used as a transparent
electrode material which is necessary for liquid crystal displays
and the like.
Some metal oxides have semiconductor characteristics. For example,
metal oxides having semiconductor characteristics include tungsten
oxide, tin oxide, indium oxide, zinc oxide, and the like, and thin
film transistors in each of which a channel formation region is
formed using such a metal oxide having semiconductor
characteristics are already known (see Patent Documents 1 to 4, and
Non-Patent Document 1).
Further, not only single-component oxides but also multi-component
oxides are known as metal oxides. For example,
InGaO.sub.3(ZnO).sub.m (m: natural number) having a homologous
series is known as a multi-component oxide semiconductor including
In, Ga, and Zn (see Non-Patent Documents 2 to 4).
Further, it is confirmed that an oxide semiconductor including an
In--Ga--Zn-based oxide as described above is applicable to a
channel layer of a thin film transistor (see Patent Document 5, and
Non-Patent Documents 5 and 6).
REFERENCE
Patent Document
[Patent Document 1] Japanese Published Patent Application No.
S60-198861 [Patent Document 2] Japanese Published Patent
Application No. H8-264794 [Patent Document 3] Japanese Translation
of PCT International Application No. H11-505377 [Patent Document 4]
Japanese Published Patent Application No. 2000-150900 [Patent
Document 5] Japanese Published Patent Application No.
2004-103957
Non-Patent Document
[Non-Patent Document 1] M. W. Prins, K. O. Grosse-Holz, G. Muller,
J. F. M. Cillessen, J. B. Giesbers, R. P. Weening, and R. M. Wolf,
"A ferroelectric transparent thin-film transistor," Appl. Phys.
Lett., 17 Jun. 1996, Vol. 68, pp. 3650-3652 [Non-Patent Document 2]
M. Nakamura, N. Kimizuka, and T. Mohri, "The Phase Relations in the
In.sub.2O.sub.3--Ga.sub.2ZnO.sub.4--ZnO System at 1350.degree. C.",
J. Solid State Chem., 1991, Vol. 93, pp. 298-315 [Non-Patent
Document 3] N. Kimizuka, M. Isobe, and M. Nakamura, "Syntheses and
Single-Crystal Data of Homologous Compounds,
In.sub.2O.sub.3(ZnO).sub.m, (m=3, 4, and 5),
InGaO.sub.3(ZnO).sub.3, and Ga.sub.2O.sub.3(ZnO).sub.m (m=7, 8, 9,
and 16) in the In.sub.2O.sub.3--ZnGa.sub.2O.sub.4--ZnO System", J.
Solid State Chem., 1995, Vol. 116, pp. 170-178 [Non-Patent Document
4] M. Nakamura, N. Kimizuka, T. Mohri, and M. Isobe, "Syntheses and
crystal structures of new homologous compounds, indium iron zinc
oxides (InFeO3(ZnO).sub.m) (m: natural number) and related
compounds", KOTAI BUTSURI (SOLID STATE PHYSICS), 1993, Vol. 28, No.
5, pp. 317-327 [Non-Patent Document 5] K. Nomura, H. Ohta, K. Ueda,
T. Kamiya, M. Hirano, and H. Hosono, "Thin-film transistor
fabricated in single-crystalline transparent oxide semiconductor",
SCIENCE, 2003, Vol. 300, pp. 1269-1272 [Non-Patent Document 6] K.
Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono,
"Room-temperature fabrication of transparent flexible thin-film
transistors using amorphous oxide semiconductors", NATURE, 2004,
Vol. 432, pp. 488-492
DISCLOSURE OF INVENTION
It is an object to manufacture and provide a highly reliable
semiconductor device including a thin film transistor having stable
electric characteristics.
In a method for manufacturing a semiconductor device which includes
a thin film transistor where a semiconductor layer including a
channel formation region and source and drain regions are formed
using an oxide semiconductor layer, heat treatment (heat treatment
for dehydration or dehydrogenation) is performed so as to improve
the purity of the oxide semiconductor layer and reduce impurities
such as moisture. In addition, not only impurities such as moisture
in the oxide semiconductor layer but also those existing in a gate
insulating layer are reduced, and impurities such as moisture
existing in interfaces between the oxide semiconductor layer and
films provided over and under and in contact with the oxide
semiconductor layer are reduced.
In this specification, an oxide semiconductor film which is used
for a semiconductor layer including a channel formation region is
referred to as a first oxide semiconductor film (a first oxide
semiconductor layer), and an oxide semiconductor film which is used
for source and drain regions is referred to as a second oxide
semiconductor film (a second oxide semiconductor layer).
In order to reduce impurities such as moisture, the first oxide
semiconductor film and the second oxide semiconductor film are
formed and then subjected to heat treatment at 200.degree. C. or
higher, preferably higher than or equal to 400.degree. C. and lower
than or equal to 600.degree. C., under an inert gas atmosphere of
nitrogen or a rare gas (such as argon or helium) or under reduced
pressure, with the first oxide semiconductor film and the second
oxide semiconductor film exposed. Thus, moisture included in the
first oxide semiconductor film and the second oxide semiconductor
film is reduced. After the heating, the oxide semiconductor films
are slowly cooled in the range of higher than or equal to room
temperature and lower than 100.degree. C. under an oxygen
atmosphere.
With the use of the first oxide semiconductor film and the second
oxide semiconductor film in which moisture is reduced by heat
treatment under an inert gas atmosphere of nitrogen or argon or
under reduced pressure, electric characteristics of a thin film
transistor is improved and a thin film transistor having both high
productivity and high performance is realized.
FIG. 29 shows results of performing thermal desorption spectroscopy
(TDS) measurement using a thermal desorption spectrometer on a
plurality of samples on which heat treatment was performed under a
nitrogen atmosphere under different heating-temperature
conditions.
The thermal desorption spectrometer is an apparatus for detecting
and identifying, using a quadrupole mass spectrometer, a gas
component which is discharged or generated from a sample when the
sample is heated and the temperature thereof is increased in high
vacuum. With the thermal desorption spectroscopy, a gas and a
molecule discharged from a surface or the inside of a sample can be
observed. A thermal desorption spectrometer manufactured by ESCO,
Ltd. (product name: EMD-WA1000S) was used. As for the measurement
condition, the rate of temperature rise was set at approximately
10.degree. C./min, the SEM voltage was set to 1500 V, the dwell
time was 0.2 (sec), and the number of channels to be used was 23.
In addition, during the measurement, the pressure was at a degree
of vacuum of approximately 1.times.10.sup.-7 (Pa). Note that the
ionization coefficient, the fragmentation coefficient, the
pass-through coefficient, and the pumping rate of H.sub.2O were
respectively 1.0, 0.805, 1.56, and 1.0.
FIG. 29 is a graph showing results of TDS measurement in terms of
H.sub.2O, where the following samples are compared: a sample
(sample 1) in which an In--Ga--Zn--O-based non-single-crystal film
was formed to a thickness of 50 nm over a glass substrate; a sample
(sample 4) on which heat treatment was performed at 250.degree. C.
for 1 hour under a nitrogen atmosphere; a sample (sample 3) on
which heat treatment was performed at 350.degree. C. for 1 hour
under a nitrogen atmosphere; a sample (sample 5) on which heat
treatment was performed at 450.degree. C. for 1 hour under a
nitrogen atmosphere; and a sample (sample 6) on which heat
treatment was performed at 350.degree. C. for 10 hours under a
nitrogen atmosphere. The results in FIG. 29 indicate that the
higher the heating temperature under a nitrogen atmosphere is, the
more impurities such as moisture (H.sub.2O) which are discharged
from the In--Ga--Zn--O-based non-single-crystal film are
reduced.
In the graph of FIG. 29, a first peak showing discharge of
impurities such as moisture (H.sub.2O) can be observed in the
vicinity of 200.degree. C. to 250.degree. C., and a second peak
showing discharge of impurities such as moisture (H.sub.2O) can be
observed at 300.degree. C. or higher.
Note that even when the sample which on which heat treatment was
performed at 450.degree. C. under a nitrogen atmosphere is left at
room temperature for approximately one week in air, discharge of
moisture at 200.degree. C. or higher is not observed. Accordingly,
it is found that the In--Ga--Zn--O-based non-single-crystal film is
stabilized by the heat treatment.
Further, TDS measurement was performed in terms of each of H, O,
OH, H.sub.2, O.sub.2, N, N.sub.2, and Ar in addition to H.sub.2O.
It was possible that a peak be clearly observed in terms of each of
H.sub.2O, H, O, and OH but not in terms of H.sub.2, O.sub.2, N,
N.sub.2, and Ar. Each sample had a structure in which an
In--Ga--Zn--O-based non-single-crystal film was formed to a
thickness of 50 nm over a glass substrate, and heat conditions were
set as follows: at 250.degree. C. for 1 hour under a nitrogen
atmosphere; at 350.degree. C. for 1 hour under a nitrogen
atmosphere; at 350.degree. C. for 10 hours under a nitrogen
atmosphere; and at 450.degree. C. for 1 hour under a nitrogen
atmosphere. A glass substrate alone and an In--Ga--Zn--O-based
non-single-crystal film on which heat treatment was not performed
were each subjected to the measurement as a comparative example.
FIG. 30, FIG. 31, FIG. 32, and FIG. 33 show results of TDS
measurement in terms of H, O, OH, and H.sub.2, respectively. Note
that the oxygen density under a nitrogen atmosphere in the above
heat conditions was 20 ppm or lower.
According to the above results, it is found that moisture is mainly
discharged by heat treatment of an In--Ga--Zn--O-based
non-single-crystal film. In other words, discharge of moisture
(H.sub.2O) from the In--Ga--Zn--O-based non-single-crystal film is
mainly caused due to heat treatment, and a product generated by
decomposition of a water molecule has an influence on values of TDS
measurement in terms of H, O, and OH which are respectively shown
in FIG. 30, FIG. 31, and FIG. 32. Note that an In--Ga--Zn--O-based
non-single-crystal film is considered to include hydrogen and OH;
therefore, these are also discharged by heat treatment.
In this specification, heat treatment under an inert gas atmosphere
of nitrogen or a rare gas (such as argon or helium) or under
reduced pressure is referred to as heat treatment for dehydration
or dehydrogenation. In this specification, "dehydrogenation" does
not refer to elimination of only H.sub.2 by heat treatment, and
"dehydration or dehydrogenation" also refers to elimination of H,
OH, and the like for convenience.
After impurities (such as H.sub.2O, H, or OH) included in the oxide
semiconductor layer are reduced by the heat treatment under an
inert gas atmosphere so that the carrier density is increased, slow
cooling is performed under an oxygen atmosphere. After the slow
cooling, for example, an oxide insulating film is formed in contact
with the oxide semiconductor layer; accordingly, the carrier
density of the oxide semiconductor layer is reduced and thus
reliability is increased.
The resistance of the first oxide semiconductor film and the second
oxide semiconductor film is reduced (the carrier density is
increased, preferably to 1.times.10.sup.18/cm.sup.3 or higher) by
the heat treatment under a nitrogen atmosphere. Thus, the first
oxide semiconductor film and the second oxide semiconductor film
each of whose resistance is reduced can be formed. The first oxide
semiconductor film and the second oxide semiconductor film each of
whose resistance is reduced are processed through an etching step
to form a first oxide semiconductor layer and a second oxide
semiconductor layer, and further processed through an etching step
to form a semiconductor layer and source and drain regions.
After that, an oxide insulating film is formed in contact with the
first oxide semiconductor layer whose resistance is reduced,
whereby in the first oxide semiconductor layer whose resistance is
reduced, the resistance of at least a region in contact with the
oxide insulating film is increased (carrier density is reduced,
preferably to lower than 1.times.10.sup.18/cm.sup.3); thus, an
oxide semiconductor region whose resistance is increased can be
formed. It is important to increase and reduce the carrier density
of the first oxide semiconductor film and the second oxide
semiconductor film by heating under an inert gas atmosphere (or
under reduced pressure), slow cooling under an oxygen atmosphere,
formation of the oxide insulating film, and the like in a
manufacturing process of a semiconductor device. In other words,
the I-type first oxide semiconductor film and the i-type second
oxide semiconductor film are subjected to heat treatment for
dehydration or dehydrogenation to be oxygen-deficiency type, that
is, n-type (such as n.sup.- or n.sup.+-type) oxide semiconductor
films, and then the oxide insulating film is formed so that the
first oxide semiconductor layer becomes oxygen-excess type, that
is, an i-type oxide semiconductor layer. Accordingly, it is
possible to manufacture and provide a semiconductor device
including a highly reliable thin film transistor having favorable
electric characteristics.
Note that as the oxide insulating film formed to be in contact with
the first oxide semiconductor layer whose resistance is reduced, an
inorganic insulating film which blocks impurities such as moisture,
a hydrogen ion, and OH.sup.- is used. Specifically a silicon oxide
film or a silicon nitride oxide film is used.
Further, after the oxide insulating film serving as a protective
film is formed over the semiconductor layer and the source and
drain regions, second heating may be performed. When the second
heating is performed after the formation of the oxide insulating
film serving as a protective film over the semiconductor layer and
the source and drain regions, variation in electric characteristics
of the thin film transistor can be reduced.
In one embodiment of the structure of the invention disclosed in
this specification, a gate electrode layer is formed; a gate
insulating layer is formed over the gate electrode layer; a first
oxide semiconductor film is formed over the gate insulating layer;
a second oxide semiconductor film is formed over the first oxide
semiconductor film; the first oxide semiconductor film and the
second oxide semiconductor film are heated to be dehydrated or
dehydrogenated and then slowly cooled under an oxygen atmosphere;
the first oxide semiconductor film and the second oxide
semiconductor film which are slowly cooled under the oxygen
atmosphere are selectively etched to form a first oxide
semiconductor layer and a second oxide semiconductor layer; a
conductive film is formed over the first oxide semiconductor layer
and the second oxide semiconductor layer; the first oxide
semiconductor layer, the second oxide semiconductor layer, and the
conductive film are selectively etched to form a semiconductor
layer, a source region, a drain region, a source electrode layer,
and a drain electrode layer; and an oxide insulating film which is
in contact with part of the semiconductor layer is formed over the
gate insulating layer, the semiconductor layer, the source region,
the drain region, the source electrode layer, and the drain
electrode layer so that carrier density is reduced.
In another embodiment of the structure of the invention disclosed
in this specification, a gate electrode layer is formed; a gate
insulating layer is formed over the gate electrode layer; a first
oxide semiconductor film is formed over the gate insulating layer;
a second oxide semiconductor film is formed over the first oxide
semiconductor film; the first oxide semiconductor film and the
second oxide semiconductor film are heated under an inert gas
atmosphere so that carrier density is increased and then slowly
cooled under an oxygen atmosphere; the first oxide semiconductor
film and the second oxide semiconductor film which are slowly
cooled under the oxygen atmosphere are selectively etched to form a
first oxide semiconductor layer and a second oxide semiconductor
layer; a conductive film is formed over the first oxide
semiconductor layer and the second oxide semiconductor layer; the
first oxide semiconductor layer, the second oxide semiconductor
layer, and the conductive film are selectively etched to form a
semiconductor layer, a source region, a drain region, a source
electrode layer, and a drain electrode layer; and an oxide
insulating film which is in contact with part of the semiconductor
layer is formed over the gate insulating layer, the semiconductor
layer, the source region, the drain region, the source electrode
layer, and the drain electrode layer so that carrier density is
reduced.
In another embodiment of the structure of the invention disclosed
in this specification, a gate electrode layer is formed; a gate
insulating layer is formed over the gate electrode layer; a first
oxide semiconductor film is formed over the gate insulating layer;
a second oxide semiconductor film is formed over the first oxide
semiconductor film; the first oxide semiconductor film and the
second oxide semiconductor film are heated under reduced pressure
so that carrier density is increased and then slowly cooled under
an oxygen atmosphere; the first oxide semiconductor film and the
second oxide semiconductor film which are slowly cooled under the
oxygen atmosphere are selectively etched to form a first oxide
semiconductor layer and a second oxide semiconductor layer; a
conductive film is formed over the first oxide semiconductor layer
and the second oxide semiconductor layer; the first oxide
semiconductor layer, the second oxide semiconductor layer, and the
conductive film are selectively etched to form a semiconductor
layer, a source region, a drain region, a source electrode layer,
and a drain electrode layer; and an oxide insulating film which is
in contact with part of the semiconductor layer is formed over the
gate insulating layer, the semiconductor layer, the source region,
a drain region, a source electrode layer, and a drain electrode
layer so that carrier density is reduced.
For an oxide semiconductor layer which can be used as the
semiconductor layer and the source and drain regions, an oxide
material having semiconductor characteristics may be used. For
example, thin films expressed by InMO.sub.3(ZnO), (m>0) are
formed, and a thin film transistor using the thin films as a
semiconductor layer and source and drain regions is manufactured.
Note that M denotes one metal element or a plurality of metal
elements selected from Ga, Fe, Ni, Mn, and Co. For example, M
denotes Ga in some cases; meanwhile, M denotes the above metal
element such as Ni or Fe in addition to Ga (Ga and Ni or Ga and Fe)
in other cases. Further, the above oxide semiconductor may include
Fe or Ni, another transitional metal element, or an oxide of the
transitional metal as an impurity element in addition to the metal
element included as M. In this specification, among the oxide
semiconductors whose composition formulas are expressed by
InMO.sub.3 (ZnO).sub.m (m>0), an oxide semiconductor which
includes Ga as M is referred to as an In--Ga--Zn--O-based oxide
semiconductor, and a thin film of the In--Ga--Zn--O-based oxide
semiconductor is also referred to as an In--Ga--Zn--O-based
non-single-crystal film.
As the oxide semiconductor which is applied to the oxide
semiconductor layer, any of the following oxide semiconductors can
be applied in addition to the above: an In--Sn--Zn--O-based oxide
semiconductor; an In--Al--Zn--O-based oxide semiconductor; a
Sn--Ga--Zn--O-based oxide semiconductor; an Al--Ga--Zn--O-based
oxide semiconductor; a Sn--Al--Zn--O-based oxide semiconductor; an
In--Zn--O-based oxide semiconductor; a Sn--Zn--O-based oxide
semiconductor; an Al--Zn--O-based oxide semiconductor; an
In--O-based oxide semiconductor; a Sn--O-based oxide semiconductor;
and a Zn--O-based oxide semiconductor. In addition, the above oxide
semiconductor layer may include silicon oxide. Silicon oxide
(SiO.sub.x (x>0)), which hinders crystallization, included in
the oxide semiconductor layer can suppress crystallization of the
oxide semiconductor layer in the case where heat treatment is
performed after formation of the oxide semiconductor layer in the
manufacturing process. Note that the oxide semiconductor layer is
preferably amorphous but may be partly crystallized.
The oxide semiconductor layer is preferably an oxide semiconductor
containing In, more preferably an oxide semiconductor containing In
and Ga. In order to obtain an I-type (intrinsic) oxide
semiconductor, dehydration or dehydrogenation is effective.
In addition, the oxide semiconductor layer used as the source
region and the drain region (also referred to as n.sup.+ layers or
buffer layers) of the thin film transistor preferably has higher
conductivity (electrical conductivity) than the oxide semiconductor
layer used as a channel formation region.
Since a thin film transistor is easily broken due to static
electricity or the like, a protective circuit for protecting a
driver circuit is preferably provided over the same substrate for a
gate line or a source line. The protective circuit is preferably
formed using a non-linear element including an oxide
semiconductor.
The gate insulating layer, the first oxide semiconductor film, and
the second oxide semiconductor film may be successively treated
(also referred to as successive treatment, an in situ process, or
successive film formation) without exposure to air. By the
successive treatment without exposure to air, each interface
between the stacked layers, that is, interfaces of the gate
insulating layer, the first oxide semiconductor film, and the
second oxide semiconductor film can be formed without being
contaminated by an atmospheric component or an impurity element
floating in air, such as water or hydrocarbon. Accordingly,
variation in characteristics of the thin film transistor can be
reduced.
Note that the term "successive treatment" in this specification
means that during a process from a first treatment step using a
PCVD method or a sputtering method to a second treatment step using
a PCVD method or a sputtering method, an atmosphere in which a
substrate to be processed is disposed is not contaminated by a
contaminant atmosphere such as air, and is constantly controlled to
be vacuum or an inert gas atmosphere (a nitrogen atmosphere or a
rare gas atmosphere). By the successive treatment, treatment such
as film formation can be performed while moisture or the like is
prevented from attaching again to the cleaned substrate to be
processed.
Performing the process from the first treatment step to the second
treatment step in the same chamber is within the scope of the
successive treatment in this specification.
In addition, the following is also within the scope of the
successive treatment in this specification: in the case of
performing the process from the first treatment step to the second
treatment step in different chambers, the substrate is transferred
after the first treatment step to another chamber without exposure
to air and is subjected to the second treatment.
Note that between the first treatment step and the second treatment
step, a substrate transfer step, an alignment step, a slow-cooling
step, a step of heating or cooling the substrate to a temperature
which is necessary for the second treatment step, or the like may
be provided. Such a process is also within the scope of the
successive treatment in this specification.
A step in which liquid is used, such as a cleaning step, wet
etching, or resist formation, may be provided between the first
treatment step and the second treatment step. This case is not
within the scope of the successive treatment in this
specification.
Note that the ordinal numbers such as "first" and "second" in this
specification are used for convenience and do not denote the order
of steps and the stacking order of layers. In addition, the ordinal
numbers in this specification do not denote particular names which
specify the invention.
Moreover, as a display device including a driver circuit, a
light-emitting display device including a light-emitting element
and a display device including an electrophoretic display element,
which is also referred to as electronic paper, are given in
addition to a liquid crystal display device.
In the light-emitting display device including a light-emitting
element, a plurality of thin film transistors are included in a
pixel portion, and in the pixel portion, there is a region where a
gate electrode of a thin film transistor is connected to a source
wiring or a drain wiring of another thin film transistor. In
addition, in a driver circuit of the light-emitting display device
including a light-emitting element, there is a region where a gate
electrode of a thin film transistor is connected to a source wiring
or a drain wiring of the thin film transistor.
In this specification, a semiconductor device generally means a
device which can function by utilizing semiconductor
characteristics, and an electro-optical device, a semiconductor
circuit, and an electronic appliance are all semiconductor
devices.
A thin film transistor having stable electric characteristics can
be manufactured and provided. Accordingly, a semiconductor device
including the highly reliable thin film transistor having favorable
electric characteristics can be provided.
BRIEF DESCRIPTION OF DRAWINGS
In the accompanying drawings:
FIGS. 1A to 1C illustrate a method for manufacturing a
semiconductor device;
FIGS. 2A and 2B illustrate the method for manufacturing a
semiconductor device;
FIGS. 3A and 3B illustrate a semiconductor device;
FIGS. 4A to 4C illustrate a method for manufacturing a
semiconductor device;
FIGS. 5A to 5C illustrate the method for manufacturing a
semiconductor device;
FIGS. 6A and 6B illustrate the method for manufacturing a
semiconductor device;
FIG. 7 illustrates a semiconductor device;
FIGS. 8A1, 8A2, 8B1, and 8B2 illustrate semiconductor devices;
FIG. 9 illustrates a semiconductor device;
FIGS. 10A1, 10A2, and 10B illustrate a semiconductor device;
FIGS. 11A and 11B illustrate a semiconductor device;
FIG. 12 illustrates a pixel equivalent circuit of a semiconductor
device;
FIGS. 13A to 13C illustrate semiconductor devices;
FIGS. 14A and 14B are block diagrams of semiconductor devices;
FIG. 15 illustrates a configuration of a signal line driver
circuit;
FIG. 16 is a timing chart illustrating operation of a signal line
driver circuit;
FIG. 17 is a timing chart illustrating operation of a signal line
driver circuit;
FIG. 18 illustrates a configuration of a shift register;
FIG. 19 illustrates a connection structure of the flip-flop
illustrated in FIG. 18;
FIG. 20 illustrates a semiconductor device;
FIG. 21 shows simulation results of oxygen density of an oxide
semiconductor layer;
FIG. 22 is an external view illustrating an example of an
electronic book reader;
FIGS. 23A and 23B are external views illustrating examples of a
television set and a digital photo frame, respectively;
FIGS. 24A and 24B are external views illustrating examples of
amusement machines;
FIGS. 25A and 25B are external views respectively illustrating an
example of a portable computer and an example of a cellular
phone;
FIG. 26 illustrates a semiconductor device;
FIG. 27 illustrates a semiconductor device;
FIG. 28 is a cross-sectional view illustrating an electric
furnace;
FIG. 29 is a graph showing results of TDS measurement;
FIG. 30 is a graph showing results of TDS measurement in terms of
H;
FIG. 31 is a graph showing results of TDS measurement in terms of
O;
FIG. 32 is a graph showing results of TDS measurement in terms of
OH;
FIG. 33 is a graph showing results of TDS measurement in terms of
H.sub.2; and
FIG. 34 illustrates a structure of an oxide semiconductor layer
used for simulation.
The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with the color drawings will be provided by the Office
upon request and payment of the necessary fee.
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiments and an example will be described in detail with
reference to drawings. However, the present invention is not
limited to the following description, and it is easily understood
by those skilled in the art that various changes and modifications
can be made in modes and details without departing from the spirit
and scope of the present invention. Therefore, the present
invention should not be interpreted as being limited to the
description of the embodiments and example below. In the structures
described below, the same portions or portions having similar
functions are denoted by the same reference numerals in different
drawings, and repetitive description thereof is omitted.
Embodiment 1
A semiconductor device and a method for manufacturing the
semiconductor device will be described with reference to FIGS. 1A
to 1C, FIGS. 2A and 2B, and FIGS. 3A and 3B.
FIG. 3A is a plan view of a thin film transistor 470 included in a
semiconductor device, and FIG. 3B is a cross-sectional view taken
along line C1-C2 of FIG. 3A. The thin film transistor 470 is an
inverted staggered thin film transistor and includes, over a
substrate 400 which is a substrate having an insulating surface, a
gate electrode layer 401, a gate insulating layer 402, a
semiconductor layer 403, source and drain regions 404a and 404b,
and source and drain electrode layers 405a and 405b. In addition,
an oxide insulating film 407 is provided to cover the thin film
transistor 470 and is in contact with the semiconductor layer
403.
Heat treatment (heat treatment for dehydration or dehydrogenation)
which reduces impurities such as moisture is performed at least
after formation of a first oxide semiconductor film and a second
oxide semiconductor film which become the semiconductor layer 403
and the source and drain regions 404a and 404b, so that the
resistance of the first oxide semiconductor film and the second
oxide semiconductor film is reduced (the carrier density is
increased, preferably to 1.times.10.sup.18/cm.sup.3 or higher).
Then, the oxide insulating film 407 is formed in contact with a
first oxide semiconductor layer, whereby the resistance thereof is
increased (the carrier density is reduced, preferably to lower than
1.times.10.sup.18/cm.sup.3, more preferably to lower than or equal
to 1.times.10.sup.14/cm.sup.3). Thus, the first oxide semiconductor
layer whose resistance is increased can be used as a channel
formation region.
Further, after impurities such as moisture (H.sub.2O) are
eliminated by the heat treatment for dehydration or
dehydrogenation, slow cooling is preferably performed under an
oxygen atmosphere. After the heat treatment for dehydration or
dehydrogenation and the slow cooling under the oxygen atmosphere,
the carrier density of the first oxide semiconductor layer is
reduced by formation of the oxide insulating film to be in contact
with the first oxide semiconductor layer, which results in that
reliability of the thin film transistor 470 is increased.
Impurities such as moisture are reduced not only in the
semiconductor layer 403 and the source and drain regions 404a and
404b but also in the gate insulating layer 402 and interfaces
between the semiconductor layer 403 which is an oxide semiconductor
layer and films that are provided over and under and in contact
with the semiconductor layer 403, specifically, an interface
between the gate insulating layer 402 and the semiconductor layer
403 and an interface between the oxide insulating film 407 and the
semiconductor layer 403.
Note that the source and drain electrode layers 405a and 405b that
are in contact with the semiconductor layer 403 which is an oxide
semiconductor layer and the source and drain regions 404a and 404b
are formed using one or more materials selected from titanium,
aluminum, manganese, magnesium, zirconium, beryllium, and thorium.
Further, a stack of alloy films including any combination of the
above elements may be used.
For the semiconductor layer 403 including a channel formation
region and the source and drain regions 404a and 404b, an oxide
material having semiconductor characteristics may be used. For
example, an oxide semiconductor having a structure expressed by
InMO.sub.3(ZnO).sub.m (m>0) can be used, and in particular, an
In--Ga--Zn--O-based oxide semiconductor is preferably used. Note
that M denotes one metal element or a plurality of metal elements
selected from gallium (Ga), iron (Fe), nickel (Ni), manganese (Mn),
and cobalt (Co). For example, M denotes Ga in some cases;
meanwhile, M denotes the above metal element such as Ni or Fe in
addition to Ga (Ga and Ni or Ga and Fe) in other cases. Further,
the above oxide semiconductor may include Fe or Ni, another
transitional metal element, or an oxide of the transitional metal
as an impurity element in addition to the metal element included as
M. In this specification, among the oxide semiconductors whose
composition formulas are expressed by InMO.sub.3 (ZnO).sub.m
(m>0), an oxide semiconductor which includes at least Ga as M is
referred to as an In--Ga--Zn--O-based oxide semiconductor, and a
thin film of the In--Ga--Zn--O-based oxide semiconductor is also
referred to as an In--Ga--Zn--O-based non-single-crystal film.
As the oxide semiconductor which is applied to the oxide
semiconductor layer, any of the following oxide semiconductors can
be applied in addition to the above: an In--Sn--Zn--O-based oxide
semiconductor; an In--Al--Zn--O-based oxide semiconductor; a
Sn--Ga--Zn--O-based oxide semiconductor; an Al--Ga--Zn--O-based
oxide semiconductor; a Sn--Al--Zn--O-based oxide semiconductor; an
In--Zn--O-based oxide semiconductor; a Sn--Zn--O-based oxide
semiconductor; an Al--Zn--O-based oxide semiconductor; an
In--O-based oxide semiconductor; a Sn--O-based oxide semiconductor;
and a Zn--O-based oxide semiconductor. In addition, the above oxide
semiconductor may include silicon oxide.
The source region is provided between the semiconductor layer (also
referred to as the first oxide semiconductor layer) and the source
electrode layer, and the drain region is provided between the
semiconductor layer and the drain electrode layer. As the source
and drain regions, an oxide semiconductor layer having n-type
conductivity (also referred to as the second oxide semiconductor
layer) can be used.
In addition, it is preferable that the second oxide semiconductor
layer used as the source and drain regions 404a and 404b of the
thin film transistor be thinner and have higher conductivity
(electrical conductivity) than the first oxide semiconductor layer
used as a channel formation region.
Further, the first oxide semiconductor layer used as the channel
formation region has an amorphous structure and the second oxide
semiconductor layer used as the source and drain regions includes a
crystal grain (nanocrystal) in the amorphous structure in some
cases. The crystal grain (nanocrystal) in the second oxide
semiconductor layer used as the source and drain regions has a
diameter of 1 nm to 10 nm, typically 2 nm to 4 nm,
approximately.
In this embodiment, In--Ga--Zn--O-based non-single-crystal films
are used as the semiconductor layer 403 including the channel
formation region and the source and drain regions (also referred to
as n.sup.+ layers or buffer layers) 404a and 404b.
FIGS. 1A to 1C and FIGS. 2A and 2B are cross-sectional views
illustrating a manufacturing process of the thin film transistor
470.
The gate electrode layer 401 is provided over the substrate 400
which is a substrate having an insulating surface. An insulating
film serving as a base film may be provided between the substrate
400 and the gate electrode layer 401. The base film has a function
of preventing diffusion of an impurity element from the substrate
400, and can be formed to have a single-layer or stacked-layer
structure using one or more films selected from a silicon nitride
film, a silicon oxide film, a silicon nitride oxide film, and a
silicon oxynitride film. The gate electrode layer 401 can be formed
to have a single-layer or stacked-layer structure using a metal
material such as molybdenum, titanium, chromium, tantalum,
tungsten, aluminum, copper, neodymium, or scandium, or an alloy
material containing any of these materials as its main
component.
For example, as a stacked-layer structure of two layers of the gate
electrode layer 401, the following structures are preferable: a
two-layer structure of an aluminum layer and a molybdenum layer
stacked thereover; a two-layer structure of a copper layer and a
molybdenum layer stacked thereover; a two-layer structure of a
copper layer and a titanium nitride layer or a tantalum nitride
layer stacked thereover; and a two-layer structure of a titanium
nitride layer and a molybdenum layer. As a stacked-layer structure
of three layers, a stack of a tungsten layer or a tungsten nitride
layer, a layer of an alloy of aluminum and silicon or a layer of an
alloy of aluminum and titanium, and a titanium nitride layer or a
titanium layer is preferable.
The gate insulating layer 402 is formed over the gate electrode
layer 401.
The gate insulating layer 402 can be formed using a single layer or
stacked layers of any of a silicon oxide layer, a silicon nitride
layer, a silicon oxynitride layer, and a silicon nitride oxide
layer with a plasma CVD method, a sputtering method, or the like.
For example, a silicon oxynitride layer may be formed with a plasma
CVD method using SiH.sub.4, oxygen, and nitrogen as a
film-formation gas.
A first oxide semiconductor film 430 and a second oxide
semiconductor film 433 are formed to be stacked over the gate
insulating layer 402 (see FIG. 1A). The first oxide semiconductor
film 430 serves as a semiconductor layer which functions as a
channel formation region, and the second oxide semiconductor film
433 serves as source and drain regions.
Note that before the oxide semiconductor films are formed with a
sputtering method, reverse sputtering in which an argon gas is
introduced and plasma is generated is preferably performed to
remove dust attached to a surface of the gate insulating layer 402.
The reverse sputtering refers to a method in which, without
application of voltage to a target side, an RF power source is used
for application of voltage to a substrate side under an argon
atmosphere to generate plasma in the vicinity of the substrate so
that a surface is modified. Note that instead of an argon
atmosphere, a nitrogen atmosphere, a helium atmosphere, or the like
may be used. Alternatively, an argon atmosphere to which oxygen,
N.sub.2O, or the like is added may be used. Further alternatively,
an argon atmosphere to which Cl.sub.2, CF.sub.4, or the like is
added may be used.
As the oxide semiconductor films, In--Ga--Zn--O-based
non-single-crystal films are used. The oxide semiconductor films
are formed with a sputtering method using an In--Ga--Zn--O-based
oxide semiconductor target. The oxide semiconductor films can be
formed with a sputtering method under a rare gas (typically argon)
atmosphere, an oxygen atmosphere, or an atmosphere of a rare gas
(typically argon) and oxygen.
The gate insulating layer 402, the first oxide semiconductor film
430, and the second oxide semiconductor film 433 may be
successively formed without exposure to air. By successive film
formation without exposure to air, each interface of the stacked
layers can be formed without being contaminated by an atmospheric
component or an impurity element floating in air, such as water or
hydrocarbon. Therefore, variation in characteristics of the thin
film transistor can be reduced.
The first oxide semiconductor film 430 and the second oxide
semiconductor film 433 are subjected to heat treatment under an
atmosphere of an inert gas (such as nitrogen, helium, neon, or
argon) or under reduced pressure, and then slowly cooled under an
oxygen atmosphere (see FIG. 1B). When the heat treatment under the
above atmosphere is performed on the first oxide semiconductor film
430 and the second oxide semiconductor film 433, impurities such as
moisture or hydrogen which are included in the first oxide
semiconductor film 430 and the second oxide semiconductor film 433
can be removed.
Note that it is preferable that impurities such as moisture or
hydrogen be not included in nitrogen or a rare gas such as helium,
neon, or argon in the heat treatment. In addition, nitrogen or a
rare gas such as helium, neon, or argon which is introduced into a
heat treatment apparatus preferably has a purity of 6N (99.9999%)
or higher, more preferably 7N (99.99999%) or higher (that is, the
density of impurities is 1 ppm or lower, preferably 0.1 ppm or
lower).
Further, in the heat treatment, a heating method using an electric
furnace or an instantaneous heating method such as a gas rapid
thermal anneal (GRTA) method using a heated gas or a lamp rapid
thermal anneal (LRTA) method can be used.
Here, as one mode of the heat treatment for the first oxide
semiconductor film 430 and the second oxide semiconductor film 433,
a heating method using an electric furnace 601 is described with
reference to FIG. 28.
FIG. 28 is a schematic view of the electric furnace 601. A heater
603 is provided outside a chamber 602 and used for heating the
chamber 602. A susceptor 605 on which a substrate 604 is set is
provided in the chamber 602, and the substrate 604 is carried into
or out of the chamber 602. Further, the chamber 602 is provided
with a gas supply unit 606 and an evacuation unit 607. A gas is
introduced into the chamber 602 by the gas supply unit 606. The
evacuation unit 607 evacuates the chamber 602 or reduces the
pressure in the chamber 602. Note that the temperature rise
characteristics of the electric furnace 601 is preferably set at
higher than or equal to 0.1.degree. C./min and lower than or equal
to 20.degree. C./min. In addition, the temperature drop
characteristics of the electric furnace 601 is preferably set at
higher than or equal to 0.1.degree. C./min and lower than or equal
to 15.degree. C./min.
The gas supply unit 606 includes a gas supply source 611a, a gas
supply source 611b, a pressure adjusting valve 612a, a pressure
adjusting valve 612b, a refiner 613a, a refiner 613b, a mass flow
controller 614a, a mass flow controller 614b, a stop valve 615a,
and a stop valve 615b. In this embodiment, the refiner 613a and the
refiner 613b are preferably provided between the gas supply source
611a and the gas supply source 611b, and the chamber 602. With the
refiner 613a and the refiner 613b, impurities such as moisture or
hydrogen in a gas which is introduced into the chamber 602 from the
gas supply source 611a and the gas supply source 611b can be
removed by the refiner 613a and the refiner 613b, so that entry of
impurities such as moisture or hydrogen into the chamber 602 can be
suppressed.
In this embodiment, nitrogen or a rare gas is introduced into the
chamber 602 from the gas supply source 611a and the gas supply
source 611b so that the atmosphere in the chamber is a nitrogen or
rare gas atmosphere, and the first oxide semiconductor film 430 and
the second oxide semiconductor film 433 which are formed over the
substrate 604 are heated in the chamber 602 heated to higher than
or equal to 200.degree. C. and lower than or equal to 600.degree.
C., preferably higher than or equal to 400.degree. C. and lower
than or equal to 450.degree. C. In this manner, dehydration or
dehydrogenation of the first oxide semiconductor film 430 and the
second oxide semiconductor film 433 can be performed.
Alternatively, dehydration or dehydrogenation of the first oxide
semiconductor film 430 and the second oxide semiconductor film 433
can be performed in such a manner that, with the pressure reduced
by the evacuation unit, the first oxide semiconductor film 430 and
the second oxide semiconductor film 433 which are formed over the
substrate 604 are heated in the chamber 602 heated to higher than
or equal to 200.degree. C. and lower than or equal to 600.degree.
C., preferably higher than or equal to 400.degree. C. and lower
than or equal to 450.degree. C.
Next, introduction of nitrogen or a rare gas from the gas supply
source 611a into the chamber 602 is stopped, and further, the
heater is turned off. Then, oxygen is introduced into the chamber
602 from the gas supply source 611b, and the chamber 602 of the
heating apparatus is slowly cooled. That is, the atmosphere in the
chamber 602 is changed to an oxygen atmosphere, and the substrate
604 is slowly cooled. Here, it is preferable that impurities such
as moisture or hydrogen be not included in oxygen which is
introduced into the chamber 602 from the gas supply source 611b. In
addition, oxygen which is introduced into the chamber 602 from the
gas supply source 611b preferably has a purity of 6N (99.9999%) or
higher, more preferably 7N (99.99999%) or higher (that is, the
density of impurities in the oxygen is 1 ppm or lower, preferably
0.1 ppm or lower). By the heat treatment under an inert gas
atmosphere or under reduced pressure, the resistance of the oxide
semiconductor films is reduced (the carrier density is increased,
preferably to 1.times.10.sup.18/cm.sup.3 or higher). Thus, a first
oxide semiconductor film 434 and a second oxide semiconductor film
435 each of whose resistance is reduced can be formed.
As a result, reliability of the thin film transistor to be
completed later can be improved.
Note that when the heat treatment is performed under reduced
pressure, cooling may be performed by introducing oxygen into the
chamber 602 after the heat treatment so that the pressure returns
to atmospheric pressure.
In addition, at the same time as introduction of oxygen into the
chamber 602 from the gas supply source 611b, one of or both
nitrogen and a rare gas such as helium, neon, or argon may be
introduced into the chamber 602.
Alternatively, after the substrate 604 in the chamber 602 of the
heating apparatus is cooled to 300.degree. C., the substrate 604
may be transferred into an atmosphere of room temperature. This
results in that cooling time for the substrate 604 can be
shortened.
When the heating apparatus has multiple chambers, the heat
treatment and the cooling treatment can be performed in different
chambers. Typically, in a first chamber which is filled with
nitrogen or a rare gas and heated to higher than or equal to
200.degree. C. and lower than or equal to 600.degree. C.,
preferably higher than or equal to 400.degree. C. and lower than or
equal to 450.degree. C., the oxide semiconductor films over the
substrate are heated. Next, the substrate on which the above heat
treatment is performed is transferred, through a transfer chamber
into which nitrogen or an inert gas is introduced, to a second
chamber which is filled with oxygen and whose temperature is lower
than or equal to 100.degree. C., preferably room temperature, and
is subjected to cooling treatment. By the above process, throughput
can be improved.
After the heat treatment under an inert gas atmosphere or under
reduced pressure, slow cooling to higher than or equal to room
temperature and lower than 100.degree. C. is performed under an
oxygen atmosphere, the substrate provided with the first oxide
semiconductor film 434 and the second oxide semiconductor film 435
is taken out of the heating apparatus, and a photolithography step
is performed.
The first oxide semiconductor film 434 and the second oxide
semiconductor film 435 after the heat treatment under an inert gas
atmosphere or under reduced pressure are preferably in an amorphous
state, but may be partly crystallized.
The first oxide semiconductor film 434 and the second oxide
semiconductor film 435 are processed into a first oxide
semiconductor layer 431 and a second oxide semiconductor layer 436
which are island-like oxide semiconductor layers through the
photolithography step (see FIG. 1C).
A conductive film is formed over the gate insulating layer 402, the
first oxide semiconductor layer 431, and the second oxide
semiconductor layer 436.
As a material for the conductive film, an element selected from Al,
Cr, Ta, Ti, Mo, and W; an alloy containing any of the above
elements as its component; an alloy film containing a combination
of any of the above elements; and the like can be given.
In the case where heat treatment is performed after the formation
of the conductive film, it is preferable that the conductive film
have heat resistance enough to withstand the heat treatment. Since
Al alone has disadvantages such as low heat resistance and a
tendency to be corroded, aluminum is used in combination with a
heat-resistant conductive material. As the heat-resistant
conductive material which is used in combination with Al, any of
the following materials may be used: an element selected from
titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo),
chromium (Cr), neodymium (Nd), and scandium (Sc); an alloy
containing any of the above elements as a component; an alloy film
containing a combination of any of the above elements; and a
nitride containing any of the above elements as a component.
The first oxide semiconductor layer 431, the second oxide
semiconductor layer 436, and the conductive film are etched through
an etching step to form a first oxide semiconductor layer 432, the
source and drain regions 404a and 404b, and the source and drain
electrode layers 405a and 405b (see FIG. 2A). Note that only part
of the first oxide semiconductor layer 431 is etched, so that the
first oxide semiconductor layer 432 has a depression (a recessed
portion).
A silicon oxide film is formed as the oxide insulating film 407
with a sputtering method so as to be in contact with the first
oxide semiconductor layer 432. The oxide insulating film 407 which
is formed to be in contact with the oxide semiconductor layer whose
resistance is reduced does not contain impurities such as moisture,
a hydrogen ion, and OH.sup.- and is formed using an inorganic
insulating film which prevents the impurities from entering from
the outside. Specifically, a silicon oxide film or a silicon
nitride oxide film is used.
In this embodiment, a silicon oxide film is formed to a thickness
of 300 nm as the oxide insulating film 407. The substrate
temperature in the film formation may be higher than or equal to
room temperature and lower than or equal to 300.degree. C., and is
set at 100.degree. C. in this embodiment. Formation of the silicon
oxide film with a sputtering method can be performed under a rare
gas (typically argon) atmosphere, an oxygen atmosphere, or an
atmosphere of a rare gas (typically argon) and oxygen. Further,
either a silicon oxide target or a silicon target may be used as a
target. For example, the silicon oxide film can be formed with a
sputtering method using a silicon target under an atmosphere of
oxygen and nitrogen.
When the oxide insulating film 407 is formed with a sputtering
method, a PCVD method, or the like so as to be in contact with the
first oxide semiconductor layer 432 whose resistance is reduced, in
the first oxide semiconductor layer 432 whose resistance is
reduced, the resistance of at least a region in contact with the
oxide insulating film 407 is increased (the carrier density is
reduced, preferably to lower than 1.times.10.sup.18/cm.sup.3, more
preferably to lower than or equal to 1.times.10.sup.14/cm.sup.3);
thus, an oxide semiconductor region whose resistance is increased
can be formed. It is important to increase and reduce the carrier
density of the oxide semiconductor layer by heating under an inert
gas atmosphere (or under reduced pressure), slow cooling under an
oxygen atmosphere, formation of the oxide insulating film, and the
like in a manufacturing process of the semiconductor device. The
first oxide semiconductor layer 432 serves as the semiconductor
layer 403 including the oxide semiconductor region whose resistance
is increased. In this manner, the thin film transistor 470 can be
manufactured (see FIG. 2B).
Impurities (such as H.sub.2O, H, or OH) included in the first oxide
semiconductor film and the second oxide semiconductor film are
reduced by the above heat treatment for dehydration or
dehydrogenation so that the carrier density is increased, and then
slow cooling is performed under an oxygen atmosphere. After the
slow cooling, the first oxide semiconductor film is processed into
an island-like first oxide semiconductor layer, and an oxide
insulating film is formed in contact with the first oxide
semiconductor layer so that the carrier density of the first oxide
semiconductor layer is reduced. When the first oxide semiconductor
layer whose carrier density is reduced is used as a semiconductor
layer, reliability of the thin film transistor 470 can be
improved.
Further, after the oxide insulating film 407 is formed, the thin
film transistor 470 may be subjected to heat treatment (preferably
at higher than or equal to 150.degree. C. and lower than
350.degree. C.) under a nitrogen atmosphere or an air atmosphere
(in air). For example, heat treatment is performed at 250.degree.
C. for 1 hour under a nitrogen atmosphere. By the heat treatment,
the semiconductor layer 403 is heated while being in contact with
the oxide insulating film 407. Accordingly, variation in electric
characteristics of the thin film transistor 470 can be reduced.
There is no particular limitation on this heat treatment
(preferably at higher than or equal to 150.degree. C. and lower
than 350.degree. C.) as long as it is performed after the formation
of the oxide insulating film 407. When the heat treatment serves
also as another step such as heat treatment in formation of a resin
film or heat treatment for reducing the resistance of a transparent
conductive film, the heat treatment can be performed without
increase in the number of steps.
Embodiment 2
A semiconductor device and a method for manufacturing the
semiconductor device will be described with reference to FIG. 26.
The same portions as Embodiment 1 or portions having functions
similar to those of Embodiment 1 can be formed in a manner similar
to that of Embodiment 1, and repetitive description thereof is
omitted.
A thin film transistor 471 illustrated in FIG. 26 is an example in
which a conductive layer 408 is provided so as to overlap with the
gate electrode layer 401 and a channel region of the semiconductor
layer 403 with an insulating film interposed between the conductive
layer 408 and the semiconductor layer 403.
FIG. 26 is a cross-sectional view of the thin film transistor 471
included in a semiconductor device. The thin film transistor 471 is
a bottom-gate thin film transistor in which, over the substrate 400
which is a substrate having an insulating surface, the gate
electrode layer 401, the gate insulating layer 402, the
semiconductor layer 403, the source and drain regions 404a and
404b, the source and drain electrode layers 405a and 405b, and the
conductive layer 408 are provided. The conductive layer 408 is
provided over the oxide insulating film 407 so as to overlap with
the gate electrode layer 401.
The conductive layer 408 can be formed using a material and a
method similar to those of the gate electrode layer 401 and the
source and drain electrode layers 405a and 405b. In the case where
a pixel electrode layer is provided, the conductive layer 408 can
be formed using a material and a method similar to those of the
pixel electrode layer. In this embodiment, a stack of a titanium
film, an aluminum film, and a titanium film is used for the
conductive layer 408.
The conductive layer 408 may have potential which is the same as or
different from that of the gate electrode layer 401, and can
function as a second gate electrode layer. Further, the conductive
layer 408 may be in a floating state.
When the conductive layer 408 is provided in a position overlapping
with the semiconductor layer 403, in a bias-temperature stress test
(hereinafter referred to as a BT test) for examining reliability of
a thin film transistor, the amount of fluctuation in threshold
voltage of the thin film transistor 471 before and after the BT
test can be reduced. Especially in a -BT test in which a voltage of
-20 V is applied to a gate after the substrate temperature is
raised to 150.degree. C., fluctuation in the threshold voltage can
be suppressed.
This embodiment can be implemented in appropriate combination with
Embodiment 1.
Embodiment 3
A semiconductor device and a method for manufacturing the
semiconductor device will be described with reference to FIG. 27.
The same portions as Embodiment 1 or portions having functions
similar to those of Embodiment 1 can be formed in a manner similar
to that of Embodiment 1, and repetitive description thereof is
omitted.
A thin film transistor 472 illustrated in FIG. 27 is an example in
which a conductive layer 409 is provided so as to overlap with the
gate electrode layer 401 and a channel region of the semiconductor
layer 403 with the oxide insulating film 407 and an insulating
layer 410 interposed between the conductive layer 409 and the
semiconductor layer 403.
FIG. 27 is a cross-sectional view of the thin film transistor 472
included in a semiconductor device. The thin film transistor 472 is
a bottom-gate thin film transistor in which, over the substrate 400
which is a substrate having an insulating surface, the gate
electrode layer 401, the gate insulating layer 402, the
semiconductor layer 403, the source and drain regions 404a and
404b, the source and drain electrode layers 405a and 405b, and the
conductive layer 409 are provided. The conductive layer 409 is
provided over the oxide insulating film 407 and the insulating
layer 410 so as to overlap with the gate electrode layer 401.
In this embodiment, the insulating layer 410 which functions as a
planarization film is stacked over the oxide insulating film 407,
and an opening which reaches the source or drain electrode layer
405b is formed in the oxide insulating film 407 and the insulating
layer 410. A conductive film is formed in the opening which is
formed in the oxide insulating film 407 and the insulating layer
410 and etched into a desired shape, so that the conductive layer
409 and a pixel electrode layer 411 are formed. In this manner, the
conductive layer 409 can be formed in the step of forming the pixel
electrode layer 411, using a similar material and method. In this
embodiment, indium oxide-tin oxide alloy including silicon oxide
(an In--Sn--O-based oxide including silicon oxide) is used for the
pixel electrode layer 411 and the conductive layer 409.
Alternatively, the conductive layer 409 may be formed using a
material and a method similar to those of the gate electrode layer
401 and the source and drain electrode layers 405a and 405b.
The conductive layer 409 may have potential which is the same as or
different from that of the gate electrode layer 401, and can
function as a second gate electrode layer. Further, the conductive
layer 409 may be in a floating state.
When the conductive layer 409 is provided in a position overlapping
with the semiconductor layer 403, in a bias-temperature stress test
(hereinafter referred to as a BT test) for examining reliability of
a thin film transistor, the amount of fluctuation in threshold
voltage of the thin film transistor 472 before and after the BT
test can be reduced.
This embodiment can be implemented in appropriate combination with
any of the structures described in the other embodiments.
Embodiment 4
A manufacturing process of a semiconductor device including a thin
film transistor will be described with reference to FIGS. 4A to 4C,
FIGS. 5A to 5C, FIGS. 6A and 6B, FIG. 7, and FIGS. 8A1, 8A2, 8B1,
and 8B2.
In FIG. 4A, as a substrate 100 having a light-transmitting
property, a glass substrate of barium borosilicate glass,
aluminoborosilicate glass, or the like can be used.
Next, a conductive layer is formed entirely over a surface of the
substrate 100, and then a first photolithography process is
performed. A resist mask is formed and unnecessary portions are
removed by etching, so that wirings and an electrode (a gate wiring
including a gate electrode layer 101, a capacitor wiring 108, and a
first terminal 121) are formed. At this time, the etching is
performed so that at least end portions of the gate electrode layer
101 are tapered.
Each of the gate wiring including the gate electrode layer 101, the
capacitor wiring 108, and the first terminal 121 in a terminal
portion is preferably formed using a heat-resistant conductive
material such as an element selected from titanium (Ti), tantalum
(Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd),
and scandium (Sc); an alloy containing any of these elements as its
component; an alloy film containing a combination of any of the
above elements; or a nitride containing any of the above elements
as its component. In the case where a low-resistant conductive
material such as aluminum (Al) or copper (Cu) is used, the
low-resistant conductive material is used in combination with the
above heat-resistant conductive material because Al alone or Cu
alone has disadvantages such as low heat resistance and a tendency
to be corroded.
Next, a gate insulating layer 102 is formed entirely over a surface
of the gate electrode layer 101 (see FIG. 4A). The gate insulating
layer 102 is formed to a thickness of 50 nm to 250 nm with a
sputtering method, a PCVD method, or the like.
For example, as the gate insulating layer 102, a silicon oxide film
is formed to a thickness of 100 nm with a sputtering method.
Needless to say, the gate insulating layer 102 is not limited to
such a silicon oxide film and may be formed to have a single-layer
structure or a stacked-layer structure using another insulating
film such as a silicon oxynitride film, a silicon nitride film, an
aluminum oxide film, or a tantalum oxide film.
Next, a first oxide semiconductor film 131 (a first
In--Ga--Zn--O-based non-single-crystal film) is formed over the
gate insulating layer 102. The first oxide semiconductor film 131
is formed without exposure to air after plasma treatment, which is
advantageous in that dust or moisture is not attached to an
interface between the gate insulating layer and the semiconductor
film. Here, the first oxide semiconductor film 131 is formed under
an argon or oxygen atmosphere using an oxide semiconductor target
having a diameter of 8 inches and including In, Ga, and Zn
(In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:1), with the distance
between the substrate and the target set to 170 mm, under a
pressure of 0.4 Pa, and with a direct current (DC) power source of
0.5 kW. Note that a pulse direct current (DC) power source is
preferable because dust can be reduced and the film thickness can
be uniform. The thickness of the first oxide semiconductor film 131
is set to 5 nm to 200 nm. As the first oxide semiconductor film
131, an In--Ga--Zn--O-based non-single-crystal film is formed to a
thickness of 50 nm with a sputtering method using an
In--Ga--Zn--O-based oxide semiconductor target.
Next, a second oxide semiconductor film 136 (a second
In--Ga--Zn--O-based non-single-crystal film) is formed with a
sputtering method without exposure to air (see FIG. 4B). Here,
sputtering is performed using a target of
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:1 under film formation
conditions where the pressure is 0.4 Pa, the power is 500 W, the
film formation temperature is room temperature, and an argon gas is
introduced at a flow rate of 40 sccm. Although the target of
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:1 is used, an
In--Ga--Zn--O-based non-single-crystal film including a crystal
grain which has a size of 1 nm to 10 nm just after the film
formation is obtained in some cases. Note that it can be said that
the presence or absence of crystal grains or the density of crystal
grains can be adjusted and the diameter size can be adjusted within
the range of 1 nm to 10 nm by appropriate adjustment of the film
formation conditions of reactive sputtering, such as the
composition ratio in the target, the film formation pressure (0.1
Pa to 2.0 Pa), the power (250 W to 3000 W: 8 inches .phi.), and the
temperature (room temperature to 100.degree. C.). The second
In--Ga--Zn--O-based non-single-crystal film has a thickness of 5 nm
to 20 nm. Needless to say, the size of a crystal grain included in
the film does not exceed the film thickness. Here, the thickness of
the second In--Ga--Zn--O-based non-single-crystal film is 5 nm.
The first In--Ga--Zn--O-based non-single-crystal film is formed
under film formation conditions different from those for the second
In--Ga--Zn--O-based non-single-crystal film. For example, the first
In--Ga--Zn--O-based non-single-crystal film is formed under
conditions where the ratio of an oxygen gas flow rate to an argon
gas flow rate is higher than the ratio of an oxygen gas flow rate
to an argon gas flow rate under the film formation conditions for
the second In--Ga--Zn--O-based non-single-crystal film.
Specifically, the second In--Ga--Zn--O-based non-single-crystal
film is formed under a rare gas (such as argon or helium)
atmosphere (or an atmosphere including an oxygen gas at less than
or equal to 10% and an argon gas at greater than or equal to 90%),
and the first In--Ga--Zn--O-based non-single-crystal film is formed
under an oxygen mixed atmosphere (an oxygen gas flow rate is higher
than a rare gas flow rate).
A chamber used for formation of the second In--Ga--Zn--O-based
non-single-crystal film may be the same as or different from the
chamber in which the reverse sputtering has been performed.
Examples of a sputtering method include an RF sputtering method in
which a high-frequency power source is used as a sputtering power
source, a DC sputtering method, and a pulsed DC sputtering method
in which a bias is applied in a pulsed manner. An RF sputtering
method is mainly used in the case where an insulating film is
formed, and a DC sputtering method is mainly used in the case where
a metal film is formed.
In addition, there is also a multi-source sputtering apparatus in
which a plurality of targets of different materials can be set.
With the multi-source sputtering apparatus, films of different
materials can be formed to be stacked in the same chamber, or a
film of plural kinds of materials can be formed by electric
discharge at the same time in the same chamber.
In addition, there are a sputtering apparatus provided with a
magnet system inside the chamber and used for a magnetron
sputtering method, and a sputtering apparatus used for an ECR
sputtering method in which plasma generated with the use of
microwaves is used without using glow discharge.
Furthermore, as a film formation method with a sputtering method,
there are also a reactive sputtering method in which a target
substance and a sputtering gas component are chemically reacted
with each other during film formation to form a thin compound film
thereof, and a bias sputtering method in which voltage is also
applied to a substrate during film formation.
Next, the first oxide semiconductor film 131 and the second oxide
semiconductor film 136 are subjected to heat treatment for
dehydration or dehydrogenation. The first oxide semiconductor film
131 and the second oxide semiconductor film 136 are subjected to
heat treatment under an atmosphere of an inert gas (such as
nitrogen, helium, neon, or argon) or under reduced pressure, and
then slowly cooled under an oxygen atmosphere.
The heat treatment is preferably performed at 200.degree. C. or
higher. For example, heat treatment is performed at 450.degree. C.
for 1 hour under a nitrogen atmosphere. By this heat treatment
under a nitrogen atmosphere, the resistance of the first oxide
semiconductor film 131 and the second oxide semiconductor film 136
is reduced (the carrier density is increased, preferably to
1.times.10.sup.18/cm.sup.3 or higher) and the conductivity thereof
is increased. Thus, a first oxide semiconductor film 133 and a
second oxide semiconductor film 137 each of whose resistance is
reduced are formed (see FIG. 4C). The electrical conductivity of
the first oxide semiconductor film 133 and the second oxide
semiconductor film 137 is preferably higher than or equal to
1.times.10.sup.-1 S/cm and lower than or equal to 1.times.10.sup.2
S/cm.
Next, a second photolithography step is performed. A resist mask is
formed, and the first oxide semiconductor film 133 and the second
oxide semiconductor film 137 are etched. For example, unnecessary
portions are removed by wet etching using a mixed solution of
phosphoric acid, acetic acid, and nitric acid, so that a first
oxide semiconductor layer 134 and a second oxide semiconductor
layer 138 are formed. Note that etching here is not limited to wet
etching, and dry etching may also be performed.
As an etching gas for dry etching, a gas containing chlorine (a
chlorine-based gas such as chlorine (Cl.sub.2), boron chloride
(BCl.sub.3), silicon chloride (SiCl.sub.4), or carbon tetrachloride
(CCl.sub.4)) is preferably used.
Alternatively, a gas containing fluorine (a fluorine-based gas such
as carbon tetrafluoride (CF.sub.4), sulfur fluoride (SF.sub.6),
nitrogen fluoride (NF.sub.3), or trifluoromethane (CHF.sub.3));
hydrogen bromide (HBr); oxygen (O.sub.2); any of these gases to
which a rare gas such as helium (He) or argon (Ar) is added; or the
like can be used.
As the dry etching method, a parallel plate reactive ion etching
(RIE) method or an inductively coupled plasma (ICP) etching method
can be used. In order to etch the films into desired shapes, the
etching condition (the amount of electric power applied to a
coil-shaped electrode, the amount of electric power applied to an
electrode on a substrate side, the temperature of the electrode on
the substrate side, or the like) is adjusted as appropriate.
As an etchant used for wet etching, a mixed solution of phosphoric
acid, acetic acid, and nitric acid, or the like can be used. In
addition, ITO-07N (produced by Kanto Chemical Co., Inc.) may be
used.
Furthermore, the etchant after the wet etching is removed together
with the etched material by cleaning. The waste liquid of the
etchant including the material etched off may be purified and the
material may be reused. When a material such as indium included in
the oxide semiconductor layer is collected from the waste liquid
after the etching and reused, the resources can be efficiently used
and the cost can be reduced.
In order to obtain a desired shape by etching, the etching
conditions (such as an etchant, etching time, and temperature) are
adjusted as appropriate depending on the material.
Next, a conductive film 132 is formed using a metal material over
the first oxide semiconductor layer 134 and the second oxide
semiconductor layer 138 with a sputtering method or a vacuum
evaporation method (see FIG. 5B).
As the material of the conductive film 132, there are an element
selected from Al, Cr, Ta, Ti, Mo, and W, an alloy containing any of
these elements as a component, an alloy film containing a
combination of any of these elements, and the like.
In the case where heat treatment is performed after the formation
of the conductive film 132, the conductive film preferably has heat
resistance enough to withstand the heat treatment.
Next, a third photolithography step is performed. A resist mask is
formed, and unnecessary portions are removed by etching, so that
source and drain electrode layers 105a and 105b, a first oxide
semiconductor layer 135, source and drain regions 104a and 104b,
and a second terminal 122 are formed (see FIG. 5C). Wet etching or
dry etching is employed as an etching method at this time. For
example, when an aluminum film or an aluminum-alloy film is used as
the conductive film 132, wet etching using a mixed solution of
phosphoric acid, acetic acid, and nitric acid can be performed.
Alternatively, by wet etching using an ammonia peroxide mixture
(hydrogen peroxide:ammonia:water=5:2:2), the conductive film 132
may be etched to form the source and drain electrode layers 105a
and 105b. In the etching step, part of an exposed region of the
first oxide semiconductor layer 134 is etched, whereby the first
oxide semiconductor layer 135 is formed. Accordingly, the first
oxide semiconductor layer 135 has a region whose thickness is small
between the source and drain electrode layers 105a and 105b. In
FIG. 5C, the source and drain electrode layers 105a and 105b, the
first oxide semiconductor layer 135, and the source and drain
regions 104a and 104b are etched at one time by dry etching;
therefore, end portions of the source and drain electrode layers
105a and 105b, the first oxide semiconductor layer 135, and the
source and drain regions 104a and 104b are aligned with one
another, and a continuous structure is formed.
In the third photolithography step, the second terminal 122 which
is formed using the same material as the source and drain electrode
layers 105a and 105b is left in the terminal portion. Note that the
second terminal 122 is electrically connected to a source wiring (a
source wiring including the source and drain electrode layers 105a
and 105b).
Further, by using a resist mask which is formed using a multi-tone
mask and has regions with plural thicknesses (typically, two
different thicknesses), the number of resist masks can be reduced,
resulting in simplified process and lower costs.
Next, the resist mask is removed, and a protective insulating layer
107 is formed to cover the gate insulating layer 102, the first
oxide semiconductor layer 135, the source and drain regions 104a
and 104b, and the source and drain electrode layers 105a and 105b.
A silicon oxynitride film formed with a PCVD method is used as the
protective insulating layer 107. The silicon oxynitride film
serving as the protective insulating layer 107 is provided in
contact with an exposed region of the first oxide semiconductor
layer 135 between the source and drain electrode layers 105a and
105b, whereby the resistance of the region of the first oxide
semiconductor layer 135 which is in contact with the protective
insulating layer 107 is increased (the carrier density is reduced,
preferably to lower than 1.times.10.sup.18/cm.sup.3, more
preferably to lower than or equal to 1.times.10.sup.14/cm.sup.3).
Thus, a semiconductor layer 103 including a channel formation
region whose resistance is increased can be formed.
Through the above process, a thin film transistor 170 can be
manufactured. Note that FIG. 7 is a plan view at this stage.
Next, a fourth photolithography step is performed. A resist mask is
formed, and the protective insulating layer 107 and the gate
insulating layer 102 are etched so that a contact hole 125 reaching
the source or drain electrode layer 105b is formed. In addition, a
contact hole 127 reaching the second terminal 122 and a contact
hole 126 reaching the first terminal 121 are also formed in the
same etching step. FIG. 6A is a cross-sectional view at this
stage.
Next, the resist mask is removed, and then a transparent conductive
film is formed. The transparent conductive film is formed using
indium oxide (In.sub.2O.sub.3), indium oxide-tin oxide alloy
(In.sub.2O.sub.3--SnO.sub.2, abbreviated to ITO), or the like with
a sputtering method, a vacuum evaporation method, or the like. Such
a material is etched with a hydrochloric acid-based solution.
However, since a residue is easily generated particularly in
etching ITO, indium oxide-zinc oxide alloy (In.sub.2O.sub.3--ZnO)
may be used to improve etching processability. Alternatively,
indium oxide-tin oxide alloy including silicon oxide (an
In--Sn--O-based oxide including silicon oxide) may be used.
In addition, when a reflective electrode layer is used as a pixel
electrode layer, it can be formed using one or more kinds of
materials selected from a metal such as tungsten (W), molybdenum
(Mo), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb),
tantalum (Ta), chromium (Cr), cobalt (Co), nickel (Ni), titanium
(Ti), platinum (Pt), aluminum (Al), copper (Cu), and silver (Ag);
an alloy thereof; and a nitride thereof.
Next, a fifth photolithography step is performed. A resist mask is
formed, and unnecessary portions are removed by etching, so that a
pixel electrode layer 110 is formed.
In the fifth photolithography step, a storage capacitor is formed
with the capacitor wiring 108 and the pixel electrode layer 110, in
which the gate insulating layer 102 and the protective insulating
layer 107 in a capacitor portion are used as a dielectric.
In addition, in the fifth photolithography step, the first terminal
121 and the second terminal 122 are covered with the resist mask,
and transparent conductive films 128 and 129 are left in the
terminal portion. The transparent conductive films 128 and 129
function as electrodes or wirings connected to an FPC. The
transparent conductive film 128 formed over the first terminal 121
is a connection terminal electrode which functions as an input
terminal of a gate wiring. The transparent conductive film 129
formed over the second terminal 122 is a connection terminal
electrode which functions as an input terminal of the source
wiring.
Next, the resist mask is removed. FIG. 6B is a cross-sectional view
at this stage.
Heat treatment may be performed after the formation of the
protective insulating layer 107 or the formation of the pixel
electrode layer 110. The heat treatment may be performed at higher
than or equal to 150.degree. C. and lower than 350.degree. C. under
an air atmosphere or a nitrogen atmosphere. In the heat treatment,
the semiconductor layer 103 is heated while being in contact with
the protective insulating layer 107; accordingly, the resistance of
the semiconductor layer 103 is further increased, and thus
improvement and less variation in electric characteristics of the
transistor can be achieved. As for the heat treatment (preferably
at higher than or equal to 150.degree. C. and lower than
350.degree. C.), there is no particular limitation as long as it is
performed after the formation of the protective insulating layer
107. When the heat treatment also serves as another step such as
heat treatment in formation of a resin film or heat treatment for
reducing the resistance of a transparent conductive film, the heat
treatment can be performed without increase in the number of
steps.
Further, FIGS. 8A1 and 8A2 are a cross-sectional view of a gate
wiring terminal portion at this stage and a plan view thereof,
respectively. FIG. 8A1 is a cross-sectional view taken along line
E1-E2 of FIG. 8A2. In FIG. 8A1, a transparent conductive film 155
formed over a protective insulating film 154 is a connection
terminal electrode which functions as an input terminal.
Furthermore, in the terminal portion of FIG. 8A1, a first terminal
151 formed using the same material as the gate wiring and a
connection electrode layer 153 formed using the same material as
the source wiring overlap with each other with a gate insulating
layer 152 interposed therebetween, and are electrically connected
to each other through the transparent conductive film 155. Note
that a portion where the transparent conductive film 128 and the
first terminal 121 are in contact with each other in FIG. 6B
corresponds to a portion where the transparent conductive film 155
and the first terminal 151 are in contact with each other in FIG.
8A1.
FIGS. 8B1 and 8B2 are respectively a cross-sectional view and a
plan view of a source wiring terminal portion which is different
from that illustrated in FIG. 6B. FIG. 8B1 is a cross-sectional
view taken along line F1-F2 of FIG. 8B2. In FIG. 8B1, the
transparent conductive film 155 formed over the protective
insulating film 154 is a connection terminal electrode which
functions as an input terminal. Furthermore, in the terminal
portion of FIG. 8B1, an electrode layer 156 formed using the same
material as the gate wiring is located below and overlaps with a
second terminal 150 which is electrically connected to the source
wiring with the gate insulating layer 152 interposed therebetween.
The electrode layer 156 is not electrically connected to the second
terminal 150, and a capacitor for preventing noise or static
electricity can be formed if the potential of the electrode layer
156 is set to a potential different from that of the second
terminal 150, such as floating, GND, or 0 V. The second terminal
150 is electrically connected to the transparent conductive film
155 with the protective insulating film 154 interposed
therebetween.
A plurality of gate wirings, source wirings, and capacitor wirings
are provided depending on the pixel density. Also in the terminal
portion, the first terminal at the same potential as the gate
wiring, the second terminal at the same potential as the source
wiring, the third terminal at the same potential as the capacitor
wiring, and the like are each arranged in plurality. The number of
each of the terminals may be any number, and the number of the
terminals may be determined by a practitioner as appropriate.
Through these five photolithography steps, the storage capacitor
and a pixel thin film transistor portion including the thin film
transistor 170 which is a bottom-gate thin film transistor having a
staggered structure can be completed using the five photomasks. By
disposing the thin film transistor and the storage capacitor in
each pixel of a pixel portion in which pixels are arranged in
matrix, one of substrates for manufacturing an active matrix
display device can be obtained. In this specification, such a
substrate is referred to as an active matrix substrate for
convenience.
In the case of manufacturing an active matrix liquid crystal
display device, an active matrix substrate and a counter substrate
provided with a counter electrode are fixed to each other with a
liquid crystal layer interposed therebetween. Note that a common
electrode which is electrically connected to the counter electrode
on the counter substrate is provided over the active matrix
substrate, and a fourth terminal which is electrically connected to
the common electrode is provided in the terminal portion. The
fourth terminal is a terminal for setting the common electrode at a
fixed potential such as GND or 0 V.
Alternatively, a storage capacitor may be formed with a pixel
electrode which overlaps with a gate wiring of an adjacent pixel,
with a protective insulating film and a gate insulating layer
interposed therebetween, without provision of the capacitor
wiring.
In an active matrix liquid crystal display device, pixel electrodes
arranged in matrix are driven so that a display pattern is formed
on a screen. Specifically, voltage is applied between a selected
pixel electrode and a counter electrode corresponding to the pixel
electrode, so that a liquid crystal layer provided between the
pixel electrode and the counter electrode is optically modulated
and this optical modulation is recognized as a display pattern by
an observer.
In displaying moving images, a liquid crystal display device has a
problem in that a long response time of liquid crystal molecules
themselves causes afterimages or blurring of moving images. In
order to improve moving image characteristics of a liquid crystal
display device, there is a driving technique called black insertion
by which a black image is displayed on the whole screen every other
frame period.
Alternatively, a driving technique called double-frame rate driving
may be employed in which a vertical synchronizing frequency is 1.5
times or more, preferably, 2 times or more as high as a usual
vertical synchronizing frequency to improve the moving-image
characteristics.
Further, in order to improve moving image characteristics of a
liquid crystal display device, there is another driving technique
in which a surface light source including a plurality of LED
(light-emitting diode) light sources or a plurality of EL light
sources is used as a backlight, and each light source included in
the surface light source is independently driven in a pulsed manner
in one frame period. As the surface light source, three or more
kinds of LEDs may be used and an LED emitting white light may be
used. Since a plurality of LEDs can be controlled independently,
the light emission timing of LEDs can be synchronized with the
timing at which a liquid crystal layer is optically modulated.
According to this driving technique, LEDs can be partly turned off;
therefore, an effect of reducing power consumption can be obtained
particularly in the case of displaying an image having a large part
on which black is displayed.
By combining these driving techniques, the display characteristics
of a liquid crystal display device, such as moving-image
characteristics, can be improved as compared to those of
conventional liquid crystal display devices.
The n-channel transistor disclosed in this specification includes
an oxide semiconductor film used for a channel formation region and
has favorable dynamic characteristics; thus, it can be combined
with these driving techniques.
In manufacturing a light-emitting display device, one electrode
(also referred to as a cathode) of an organic light-emitting
element is set to a low power supply potential such as GND or 0 V;
thus, a terminal portion is provided with a fourth terminal for
setting the cathode to a low power supply potential such as GND or
0 V. Further, in manufacturing a light-emitting display device, a
power supply line is provided in addition to a source wiring and a
gate wiring. Accordingly, the terminal portion is provided with a
fifth terminal which is electrically connected to the power supply
line.
In manufacturing a light-emitting display device, a partition wall
including an organic resin layer is provided between organic
light-emitting elements in some cases. In that case, heat treatment
performed on the organic resin layer can also serve as the heat
treatment which increases the resistance of the semiconductor layer
103 so that improvement and less variation in electric
characteristics of the transistor are achieved.
The use of an oxide semiconductor for a thin film transistor leads
to reduction in manufacturing cost. In particular, by the heat
treatment for dehydration or dehydrogenation, impurities such as
moisture are reduced and the purity of the oxide semiconductor film
is increased. Therefore, a semiconductor device including a highly
reliable thin film transistor having favorable electric
characteristics can be manufactured without using an ultrapure
oxide semiconductor target or a special sputtering apparatus in
which dew point in a film-formation chamber is reduced.
Since the semiconductor layer in the channel formation region is a
region whose resistance is increased, electric characteristics of
the thin film transistor are stabilized, and increase in off
current or the like can be prevented. Accordingly, a semiconductor
device including the highly reliable thin film transistor having
favorable electric characteristics can be provided.
This embodiment can be implemented in appropriate combination with
any of the structures described in the other embodiments.
Embodiment 5
An example will be described below, in which at least part of a
driver circuit and a thin film transistor arranged in a pixel
portion are formed over the same substrate in a display device
which is one example of a semiconductor device.
The thin film transistor to be disposed in the pixel portion is
formed in accordance with any of Embodiments 1 to 4. Further, the
thin film transistor described in any of Embodiments 1 to 4 is an
n-channel TFT, and thus part of a driver circuit that can include
an n-channel TFT among driver circuits is formed over the same
substrate as the thin film transistor of the pixel portion.
FIG. 14A illustrates an example of a block diagram of an active
matrix liquid crystal display device which is an example of a
semiconductor device. The display device illustrated in FIG. 14A
includes, over a substrate 5300, a pixel portion 5301 including a
plurality of pixels that are each provided with a display element,
a scan line driver circuit 5302 that selects each pixel, and a
signal line driver circuit 5303 that controls a video signal input
to the selected pixel.
The pixel portion 5301 is connected to the signal line driver
circuit 5303 by a plurality of signal lines S1 to Sm (not
illustrated) which extend in a column direction from the signal
line driver circuit 5303, and to the scan line driver circuit 5302
by a plurality of scan lines G1 to Gn (not illustrated) that extend
in a row direction from the scan line driver circuit 5302. The
pixel portion 5301 includes a plurality of pixels (not illustrated)
arranged in matrix so as to correspond to the signal lines S1 to Sm
and the scan lines G1 to Gn. Each pixel is connected to a signal
line Sj (one of the signal lines S1 to Sm) and a scan line Gi (one
of the scan lines G1 to Gn).
In addition, the thin film transistor described in any of
Embodiments 1 to 4 is an n-channel TFT, and a signal line driver
circuit including the n-channel TFT is described with reference to
FIG. 15.
The signal line driver circuit illustrated in FIG. 15 includes a
driver IC 5601, switch groups 5602_1 to 5602_M, a first wiring
5611, a second wiring 5612, a third wiring 5613, and wirings 5621_1
to 5621_M. Each of the switch groups 5602_1 to 5602_M includes a
first thin film transistor 5603a, a second thin film transistor
5603b, and a third thin film transistor 5603c.
The driver IC 5601 is connected to the first wiring 5611, the
second wiring 5612, the third wiring 5613, and the wirings 5621_1
to 5621_M. Each of the switch groups 5602_1 to 5602_M is connected
to the first wiring 5611, the second wiring 5612, and the third
wiring 5613, and the wirings 5621_1 to 5621_M are connected to the
switch groups 5602_1 to 5602_M, respectively. Each of the wirings
5621_1 to 5621_M is connected to three signal lines via the first
thin film transistor 5603a, the second thin film transistor 5603b,
and the third thin film transistor 5603c. For example, the wiring
5621_J of the J-th column (one of the wirings 5621_1 to 5621_M) is
connected to a signal line Sj-1, a signal line Sj, and a signal
line Sj+1 via the first thin film transistor 5603a, the second thin
film transistor 5603b, and the third thin film transistor 5603c
which are included in the switch group 5602_J.
A signal is inputted to each of the first wiring 5611, the second
wiring 5612, and the third wiring 5613.
Note that the driver IC 5601 is preferably formed over a single
crystal substrate. Further, the switch groups 5602_1 to 5602_M are
preferably formed over the same substrate as the pixel portion.
Therefore, the driver IC 5601 and the switch groups 5602_1 to
5602_M are preferably connected through an FPC or the like.
Next, operation of the signal line driver circuit illustrated in
FIG. 15 is described with reference to a timing chart in FIG. 16.
The timing chart in FIG. 16 illustrates the case where the scan
line Gi of the i-th row is selected. A selection period of the scan
line Gi of the i-th row is divided into a first sub-selection
period T1, a second sub-selection period T2, and a third
sub-selection period T3. In addition, the signal line driver
circuit in FIG. 15 operates similarly to that in FIG. 16 even when
a scan line of another row is selected.
Note that the timing chart in FIG. 16 illustrates the case where
the wiring 5621_J of the J-th column is connected to the signal
line Sj-1, the signal line Sj, and the signal line Sj+1 via the
first thin film transistor 5603a, the second thin film transistor
5603b, and the third thin film transistor 5603c.
Note that the timing chart in FIG. 16 illustrates timing at which
the scan line Gi of the i-th row is selected, timing 5703a at which
the first thin film transistor 5603a is turned on/off, timing 5703b
at which the second thin film transistor 5603b is turned on/off,
timing 5703c at which the third thin film transistor 5603c is
turned on/off, and a signal 5721_J inputted to the wiring 5621_J of
the J-th column.
In the first sub-selection period T1, the second sub-selection
period T2, and the third sub-selection period T3, different video
signals are inputted to the wirings 5621_1 to 5621_M. For example,
a video signal inputted to the wiring 5621_J in the first
sub-selection period T1 is inputted to the signal line Sj-1, a
video signal inputted to the wiring 5621_J in the second
sub-selection period T2 is inputted to the signal line Sj, and a
video signal inputted to the wiring 5621_J in the third
sub-selection period T3 is inputted to the signal line Sj+1. In
addition, the video signals inputted to the wiring 5621_J in the
first sub-selection period T1, the second sub-selection period T2,
and the third sub-selection period T3 are denoted by Data_j-1,
Data_j, and Data_j+1.
As illustrated in FIG. 16, in the first sub-selection period T1,
the first thin film transistor 5603a is turned on, and the second
thin film transistor 5603b and the third thin film transistor 5603c
are turned off. At this time, Data_j-1 inputted to the wiring
5621_J is inputted to the signal line Sj-1 via the first thin film
transistor 5603a. In the second sub-selection period T2, the second
thin film transistor 5603b is turned on, and the first thin film
transistor 5603a and the third thin film transistor 5603c are
turned off. At this time, Data_j inputted to the wiring 5621_J is
inputted to the signal line Sj via the second thin film transistor
5603b. In the third sub-selection period T3, the third thin film
transistor 5603c is turned on, and the first thin film transistor
5603a and the second thin film transistor 5603b are turned off. At
this time, Data_j+1 inputted to the wiring 5621_J is inputted to
the signal line Sj+1 via the third thin film transistor 5603c.
As described above, in the signal line driver circuit in FIG. 15,
by dividing one gate selection period into three, video signals can
be inputted to three signal lines from one wiring 5621 in one gate
selection period. Therefore, in the signal line driver circuit in
FIG. 15, the number of connections between the substrate provided
with the driver IC 5601 and the substrate provided with the pixel
portion can be approximately 1/3 of the number of signal lines. The
number of connections is reduced to approximately 1/3 of the number
of the signal lines, so that reliability, yield, and the like of
the signal line driver circuit in FIG. 15 can be improved.
Note that there is no particular limitation on the arrangement, the
number, a driving method, and the like of the thin film
transistors, as long as one gate selection period is divided into a
plurality of sub-selection periods and video signals are inputted
to a plurality of signal lines from one wiring in the respective
sub-selection periods as illustrated in FIG. 15.
For example, when video signals are inputted to three or more
signal lines from one wiring in each of three or more sub-selection
periods, it is only necessary to add a thin film transistor and a
wiring for controlling the thin film transistor. Note that when one
gate selection period is divided into four or more sub-selection
periods, one sub-selection period becomes shorter. Therefore, one
gate selection period is preferably divided into two or three
sub-selection periods.
As another example, one gate selection period may be divided into a
precharge period Tp, the first sub-selection period T1, the second
sub-selection period T2, and the third sub-selection period T3 as
illustrated in a timing chart in FIG. 17. Further, the timing chart
in FIG. 17 illustrates the timing at which the scan line Gi of the
i-th row is selected, timing 5803a at which the first thin film
transistor 5603a is turned on/off, timing 5803b at which the second
thin film transistor 5603b is turned on/off, timing 5803c at which
the third thin film transistor 5603c is turned on/off, and a signal
5821_J inputted to the wiring 5621_J of the J-th column. As
illustrated in FIG. 17, the first thin film transistor 5603a, the
second thin film transistor 5603b, and the third thin film
transistor 5603c are turned on in the precharge period Tp. At this
time, precharge voltage V.sub.p inputted to the wiring 5621_J is
inputted to each of the signal line Sj-1, the signal line Sj, and
the signal line Sj+1 via the first thin film transistor 5603a, the
second thin film transistor 5603b, and the third thin film
transistor 5603c. In the first sub-selection period T1, the first
thin film transistor 5603a is turned on, and the second thin film
transistor 5603b and the third thin film transistor 5603c are
turned off. At this time, Data_j-1 inputted to the wiring 5621_J is
inputted to the signal line Sj-1 via the first thin film transistor
5603a. In the second sub-selection period T2, the second thin film
transistor 5603b is turned on, and the first thin film transistor
5603a and the third thin film transistor 5603c are turned off. At
this time, Data_j inputted to the wiring 5621_J is inputted to the
signal line Sj via the second thin film transistor 5603b. In the
third sub-selection period T3, the third thin film transistor 5603c
is turned on, and the first thin film transistor 5603a and the
second thin film transistor 5603b are turned off. At this time,
Data_j+1 inputted to the wiring 5621_J is inputted to the signal
line Sj+1 via the third thin film transistor 5603c.
As described above, in the signal line driver circuit in FIG. 15 to
which the timing chart in FIG. 17 is applied, the video signal can
be written to the pixel at high speed because the signal line can
be precharged by providing a precharge period before a
sub-selection period. Note that portions of FIG. 17 which are
similar to those of FIG. 16 are denoted by common reference
numerals and detailed description of the same portions and portions
having similar functions is omitted.
Further, a configuration of a scan line driver circuit is
described. The scan line driver circuit includes a shift register.
Additionally, the scan line driver circuit may include a level
shifter or a buffer in some cases. In the scan line driver circuit,
when a clock signal (CLK) and a start pulse signal (SP) are
inputted to the shift register, a selection signal is generated.
The generated selection signal is buffered and amplified by the
buffer, and the resulting signal is supplied to a corresponding
scan line. Gate electrodes of transistors in pixels of one line are
connected to the scan line. Since the transistors in the pixels of
one line have to be turned on all at once, a buffer which can
supply large current is used.
One mode of a shift register used for part of the scan line driver
circuit will be described with reference to FIG. 18 and FIG.
19.
FIG. 18 illustrates a circuit configuration of the shift register.
The shift register illustrated in FIG. 18 includes a plurality of
flip-flops: flip-flops 5701_1 to 5701.sub.--n. The shift register
is operated with input of a first clock signal, a second clock
signal, a start pulse signal, and a reset signal.
The connection relationship of the shift register in FIG. 18 will
be described. In the i-th stage flip-flop 5701.sub.--i (one of the
flip-flops 5701_1 to 5701.sub.--n) in the shift register in FIG.
18, a first wiring 5501 illustrated in FIG. 19 is connected to a
seventh wiring 5717.sub.--i-1, a second wiring 5502 illustrated in
FIG. 19 is connected to a seventh wiring 5717.sub.--i+1, a third
wiring 5503 illustrated in FIG. 19 is connected to a seventh wiring
5717.sub.--i, and a sixth wiring 5506 illustrated in FIG. 19 is
connected to a fifth wiring 5715.
Further, a fourth wiring 5504 illustrated in FIG. 19 is connected
to a second wiring 5712 in flip-flops of odd-numbered stages, and
is connected to a third wiring 5713 in flip-flops of even-numbered
stages. A fifth wiring 5505 illustrated in FIG. 19 is connected to
a fourth wiring 5714.
Note that the first wiring 5501 of the first stage flip-flop 5701_1
which is illustrated in FIG. 19 is connected to a first wiring
5711. Moreover, the second wiring 5502 of the n-th stage flip-flop
5701.sub.--n which is illustrated in FIG. 19 is connected to a
sixth wiring 5716.
Note that the first wiring 5711, the second wiring 5712, the third
wiring 5713, and the sixth wiring 5716 may be referred to as a
first signal line, a second signal line, a third signal line, and a
fourth signal line, respectively. The fourth wiring 5714 and the
fifth wiring 5715 may be referred to as a first power supply line
and a second power supply line, respectively.
Next, FIG. 19 illustrates details of the flip-flop shown in FIG.
18. A flip-flop illustrated in FIG. 19 includes a first thin film
transistor 5571, a second thin film transistor 5572, a third thin
film transistor 5573, a fourth thin film transistor 5574, a fifth
thin film transistor 5575, a sixth thin film transistor 5576, a
seventh thin film transistor 5577, and an eighth thin film
transistor 5578. Each of the first thin film transistor 5571, the
second thin film transistor 5572, the third thin film transistor
5573, the fourth thin film transistor 5574, the fifth thin film
transistor 5575, the sixth thin film transistor 5576, the seventh
thin film transistor 5577, and the eighth thin film transistor 5578
is an n-channel transistor and is turned on when the gate-source
voltage (V.sub.gs) exceeds the threshold voltage (V.sub.th).
Next, a connection configuration of the flip-flop illustrated in
FIG. 19 will be described below.
A first electrode (one of a source electrode and a drain electrode)
of the first thin film transistor 5571 is connected to the fourth
wiring 5504. A second electrode (the other of the source electrode
and the drain electrode) of the first thin film transistor 5571 is
connected to the third wiring 5503.
A first electrode of the second thin film transistor 5572 is
connected to the sixth wiring 5506. A second electrode of the
second thin film transistor 5572 is connected to the third wiring
5503.
A first electrode of the third thin film transistor 5573 is
connected to the fifth wiring 5505, and a second electrode of the
third thin film transistor 5573 is connected to a gate electrode of
the second thin film transistor 5572. A gate electrode of the third
thin film transistor 5573 is connected to the fifth wiring
5505.
A first electrode of the fourth thin film transistor 5574 is
connected to the sixth wiring 5506, and a second electrode of the
fourth thin film transistor 5574 is connected to the gate electrode
of the second thin film transistor 5572. A gate electrode of the
fourth thin film transistor 5574 is connected to a gate electrode
of the first thin film transistor 5571.
A first electrode of the fifth thin film transistor 5575 is
connected to the fifth wiring 5505, and a second electrode of the
fifth thin film transistor 5575 is connected to the gate electrode
of the first thin film transistor 5571. A gate electrode of the
fifth thin film transistor 5575 is connected to the first wiring
5501.
A first electrode of the sixth thin film transistor 5576 is
connected to the sixth wiring 5506, and a second electrode of the
sixth thin film transistor 5576 is connected to the gate electrode
of the first thin film transistor 5571. A gate electrode of the
sixth thin film transistor 5576 is connected to the gate electrode
of the second thin film transistor 5572.
A first electrode of the seventh thin film transistor 5577 is
connected to the sixth wiring 5506, and a second electrode of the
seventh thin film transistor 5577 is connected to the gate
electrode of the first thin film transistor 5571. A gate electrode
of the seventh thin film transistor 5577 is connected to the second
wiring 5502. A first electrode of the eighth thin film transistor
5578 is connected to the sixth wiring 5506, and a second electrode
of the eighth thin film transistor 5578 is connected to the gate
electrode of the second thin film transistor 5572. A gate electrode
of the eighth thin film transistor 5578 is connected to the first
wiring 5501.
Note that the point at which the gate electrode of the first thin
film transistor 5571, the gate electrode of the fourth thin film
transistor 5574, the second electrode of the fifth thin film
transistor 5575, the second electrode of the sixth thin film
transistor 5576, and the second electrode of the seventh thin film
transistor 5577 are connected is referred to as a node 5543.
Further, the point at which the gate electrode of the second thin
film transistor 5572, the second electrode of the third thin film
transistor 5573, the second electrode of the fourth thin film
transistor 5574, the gate electrode of the sixth thin film
transistor 5576, and the second electrode of the eighth thin film
transistor 5578 are connected is referred to as a node 5544.
Note that the first wiring 5501, the second wiring 5502, the third
wiring 5503, and the fourth wiring 5504 may be referred to as a
first signal line, a second signal line, a third signal line, and a
fourth signal line, respectively. The fifth wiring 5505 and the
sixth wiring 5506 may be referred to as a first power supply line
and a second power supply line, respectively.
In addition, the signal line driver circuit and the scan line
driver circuit can be formed using only the n-channel TFTs
described in any of Embodiments 1 to 4. The n-channel TFT described
in any of Embodiments 1 to 4 have a high mobility, and thus a
driving frequency of the driver circuits can be increased. Further,
since parasitic capacitance is reduced in the n-channel TFT
described in any of Embodiments 1 to 4, high frequency
characteristics (referred to as f characteristics) is obtained. For
example, a scan line driver circuit using the n-channel TFT
described in any of Embodiments 1 to 4 can operate at high speed,
and thus a frame frequency can be increased and insertion of black
images can be realized, for example.
In addition, when the channel width of the transistor in the scan
line driver circuit is increased or a plurality of scan line driver
circuits are provided, for example, higher frame frequency can be
realized. When a plurality of scan line driver circuits are
provided, a scan line driver circuit for driving scan lines of
even-numbered rows is provided on one side and a scan line driver
circuit for driving scan lines of odd-numbered rows is provided on
the opposite side; thus, increase in frame frequency can be
realized. Furthermore, the use of the plurality of scan line driver
circuits for output of signals to the same scan line is
advantageous in increasing the size of a display device.
Further, when an active matrix light-emitting display device which
is an example of a semiconductor device is manufactured, a
plurality of thin film transistors are arranged in at least one
pixel, and thus a plurality of scan line driver circuits are
preferably arranged. FIG. 14B illustrates an example of a block
diagram of an active matrix light-emitting display device.
The light-emitting display device illustrated in FIG. 14B includes,
over a substrate 5400, a pixel portion 5401 including a plurality
of pixels each provided with a display element, a first scan line
driver circuit 5402 and a second scan line driver circuit 5404 that
select each pixel, and a signal line driver circuit 5403 that
controls a video signal input to the selected pixel.
When the video signal inputted to a pixel of the light-emitting
display device illustrated in FIG. 14B is a digital signal, a pixel
emits light or does not emit light by switching a transistor
on/off. Thus, grayscale can be displayed using an area grayscale
method or a time grayscale method. An area grayscale method refers
to a driving method in which one pixel is divided into a plurality
of sub-pixels and the respective sub-pixels are driven
independently based on video signals so that grayscale is
displayed. Further, a time grayscale method refers to a driving
method in which a period during which a pixel emits light is
controlled so that grayscale is displayed.
Since the response speed of a light-emitting element is higher than
that of a liquid crystal element or the like, the light-emitting
element is more suitable for a time grayscale method than the
liquid crystal element. Specifically, in the case of displaying
with a time grayscale method, one frame period is divided into a
plurality of sub-frame periods. Then, in accordance with video
signals, the light-emitting element in the pixel is brought into a
light-emitting state or a non-light-emitting state in each
sub-frame period. By dividing one frame period into a plurality of
sub-frame periods, the total length of time, in which a pixel
actually emits light in one frame period, can be controlled by
video signals so that grayscale can be displayed.
Note that in the example of the light-emitting display device
illustrated in FIG. 14B, when two switching TFTs are arranged in
one pixel, the first scan line driver circuit 5402 generates a
signal which is inputted to a first scan line serving as a gate
wiring of one of the two switching TFTs, and the second scan line
driver circuit 5404 generates a signal which is inputted to a
second scan line serving as a gate wiring of the other of the two
switching TFTs. However, one scan line driver circuit may generate
both the signal which is inputted to the first scan line and the
signal which is inputted to the second scan line. In addition, for
example, a possibility that a plurality of scan lines used to
control the operation of the switching element are provided in each
pixel depending on the number of the switching TFTs included in one
pixel. In this case, one scan line driver circuit may generate all
signals that are inputted to the plurality of scan lines, or a
plurality of scan line driver circuits may generate signals that
are inputted to the plurality of scan lines.
Also in the light-emitting display device, part of a driver circuit
that can include an n-channel TFT among driver circuits can be
formed over the same substrate as the thin film transistors of the
pixel portion. Alternatively, the signal line driver circuit and
the scan line driver circuit can be formed using only the n-channel
TFTs described in any of Embodiments 1 to 4.
Through the above process, a highly reliable display device as a
semiconductor device can be manufactured.
This embodiment can be implemented in appropriate combination with
any of the structures described in the other embodiments.
Embodiment 6
When a thin film transistor is manufactured and used for a pixel
portion and further for a driver circuit, a semiconductor device
having a display function (also referred to as a display device)
can be manufactured. Furthermore, when part or whole of a driver
circuit using a thin film transistor is formed over the same
substrate as a pixel portion, a system-on-panel can be
obtained.
The display device includes a display element. As the display
element, a liquid crystal element (also referred to as a liquid
crystal display element) or a light-emitting element (also referred
to as a light-emitting display element) can be used. Light-emitting
elements include, in its category, an element whose luminance is
controlled by current or voltage, and specifically include an
inorganic electroluminescent (EL) element, an organic EL element,
and the like. Furthermore, a display medium whose contrast is
changed by an electric effect, such as electronic ink, can be
used.
In addition, the display device includes a panel in which the
display element is sealed, and a module in which an IC or the like
including a controller is mounted on the panel. An embodiment of
the present invention also relates to an element substrate which
corresponds to one mode before the display element is completed in
a manufacturing process of the display device, and the element
substrate is provided with a unit for supplying current to the
display element in each of a plurality of pixels. Specifically, the
element substrate may be in a state where only a pixel electrode of
the display element is formed, a state after a conductive film to
be a pixel electrode is formed and before the conductive film is
etched to form the pixel electrode, or any of other states.
Note that a display device in this specification means an image
display device, a display device, or a light source (including a
lighting device). Furthermore, the display device also includes the
following modules in its category: a module to which a connector
such as a flexible printed circuit (FPC), a tape automated bonding
(TAB) tape, or a tape carrier package (TCP) is attached; a module
having a TAB tape or a TCP at the tip of which a printed wiring
board is provided; and a module in which an integrated circuit (IC)
is directly mounted on a display element with a chip on glass (COG)
method.
The appearance and cross section of a liquid crystal display panel
which is one embodiment of a semiconductor device will be described
with reference to FIGS. 10A1, 10A2 and 10B. FIGS. 10A1 and 10A2 are
each a plan view of a panel in which a liquid crystal element 4013
and highly reliable thin film transistors 4010 and 4011 each
including the oxide semiconductor layer, which are like the thin
film transistor described in Embodiment 4, which are formed over a
first substrate 4001 are sealed between the first substrate 4001
and a second substrate 4006 with a sealant 4005. FIG. 10B is a
cross-sectional view taken along line M-N of FIGS. 10A1 and
10A2.
The sealant 4005 is provided to surround a pixel portion 4002 and a
scan line driver circuit 4004 that are provided over the first
substrate 4001. The second substrate 4006 is provided over the
pixel portion 4002 and the scan line driver circuit 4004.
Therefore, the pixel portion 4002 and the scan line driver circuit
4004 are sealed together with a liquid crystal layer 4008, by the
first substrate 4001, the sealant 4005, and the second substrate
4006. A signal line driver circuit 4003 that is formed using a
single crystal semiconductor film or a polycrystalline
semiconductor film over a substrate separately prepared is mounted
in a region different from the region surrounded by the sealant
4005 over the first substrate 4001.
Note that there is no particular limitation on the connection
method of a driver circuit which is separately formed, and a COG
method, a wire bonding method, a TAB method, or the like can be
used. FIG. 10A1 illustrates an example of mounting the signal line
driver circuit 4003 with a COG method, and FIG. 10A2 illustrates an
example of mounting the signal line driver circuit 4003 with a TAB
method.
The pixel portion 4002 and the scan line driver circuit 4004
provided over the first substrate 4001 each include a plurality of
thin film transistors, and FIG. 10B illustrates an example of the
thin film transistor 4010 included in the pixel portion 4002 and
the thin film transistor 4011 included in the scan line driver
circuit 4004. Insulating layers 4020 and 4021 are provided over the
thin film transistors 4010 and 4011.
As the thin film transistors 4010 and 4011, a highly reliable thin
film transistor including oxide semiconductor layer like the thin
film transistor described in Embodiment 4 can be employed.
Alternatively, the thin film transistor described in any of
Embodiments 1 to 3 may be employed. In this embodiment, the thin
film transistors 4010 and 4011 are n-channel thin film
transistors.
A pixel electrode layer 4030 included in the liquid crystal element
4013 is electrically connected to the thin film transistor 4010. A
counter electrode layer 4031 of the liquid crystal element 4013 is
formed on the second substrate 4006. A portion where the pixel
electrode layer 4030, the counter electrode layer 4031, and the
liquid crystal layer 4008 overlap with one another corresponds to
the liquid crystal element 4013. Note that the pixel electrode
layer 4030 and the counter electrode layer 4031 are provided with
an insulating layer 4032 and an insulating layer 4033,
respectively, each of which functions as an alignment film. The
liquid crystal layer 4008 is sandwiched between the pixel electrode
layer 4030 and the counter electrode layer 4031 with the insulating
layers 4032 and 4033 interposed therebetween.
Note that the first substrate 4001 and the second substrate 4006
can be made of glass, metal (typically, stainless steel), ceramic,
or plastic. As plastic, a fiberglass-reinforced plastics (FRP)
plate, a polyvinyl fluoride (PVF) film, a polyester film, or an
acrylic resin film can be used. Alternatively, a sheet with a
structure in which an aluminum foil is sandwiched between PVF films
or polyester films can be used.
A columnar spacer denoted by reference numeral 4035 is obtained by
selective etching of an insulating film and is provided in order to
control the distance (a cell gap) between the pixel electrode layer
4030 and the counter electrode layer 4031. Note that a spherical
spacer may also be used. The counter electrode layer 4031 is
electrically connected to a common potential line provided over the
same substrate as the thin film transistor 4010. With the use of
the common connection portion, the counter electrode layer 4031 can
be electrically connected to the common potential line through
conductive particles provided between the pair of substrates. Note
that the conductive particles are contained in the sealant
4005.
Alternatively, liquid crystal showing a blue phase for which an
alignment film is unnecessary may be used. A blue phase is one of
the liquid crystal phases, which is generated just before a
cholesteric phase changes into an isotropic phase while temperature
of cholesteric liquid crystal is increased. Since the blue phase is
only generated within a narrow range of temperatures, a liquid
crystal composition containing a chiral agent at 5 wt % or more is
used for the liquid crystal layer 4008 in order to improve the
temperature range. The liquid crystal composition which includes
liquid crystal exhibiting a blue phase and a chiral agent has a
short response speed of 1 msec or less, has optical isotropy, which
makes the alignment process unneeded, and has a small viewing angle
dependence.
An embodiment of the present invention can also be applied to a
reflective liquid crystal display device or a semi-transmissive
liquid crystal display device, in addition to a transmissive liquid
crystal display device.
An example of the liquid crystal display device is described in
which a polarizing plate is provided on the outer surface of the
substrate (on the viewer side) and a coloring layer and an
electrode layer used for a display element are provided on the
inner surface of the substrate in this order; however, the
polarizing plate may be provided on the inner surface of the
substrate. The stacked-layer structure of the polarizing plate and
the coloring layer is not limited to that described in this
embodiment and may be set as appropriate in a manner that depends
on materials of the polarizing plate and the coloring layer or
conditions of manufacturing steps. Furthermore, a light-blocking
film serving as a black matrix may be provided.
In the thin film transistors 4010 and 4011, the insulating layer
4020 is formed as a protective insulating film so as to be in
contact with a semiconductor layer including a channel formation
region. The insulating layer 4020 may be formed using a material
and a method similar to those of the oxide insulating film 407
described in Embodiment 1. In addition, in order to reduce the
surface roughness of the thin film transistors, the thin film
transistors are covered with the insulating layer 4021 serving as a
planarizing insulating film.
In this embodiment, the insulating layer 4020 having a
stacked-layer structure is formed as the protective film. As a
first layer of the insulating layer 4020, a silicon oxide film is
formed with a sputtering method. The use of the silicon oxide film
as the protective film has the effect of preventing a hillock of an
aluminum film used for the source and drain electrode layers.
An insulating layer is formed as a second layer of the protective
film. As a second layer of the insulating layer 4020, a silicon
nitride film is formed with a sputtering method. The use of the
silicon nitride film as the protective film can prevent mobile ions
such as sodium ions from entering a semiconductor region, thereby
suppressing variations in electric characteristics of the TFTs.
After the protective film is formed, heat treatment (at 300.degree.
C. or less) may be performed under a nitrogen atmosphere or an air
atmosphere.
The insulating layer 4021 is formed as the planarizing insulating
film. As the insulating layer 4021, an organic material having heat
resistance such as polyimide, acrylic, benzocyclobutene, polyamide,
or epoxy can be used. Other than such organic materials, it is also
possible to use a low-dielectric constant material (a low-k
material), a siloxane-based resin, phosphosilicate glass (PSG),
borophosphosilicate glass (BPSG), or the like. Note that the
insulating layer 4021 may be formed by stacking a plurality of
insulating films formed using any of these materials.
Note that a siloxane-based resin is a resin formed from a
siloxane-based material as a starting material and having a
Si--O--Si bond. The siloxane-based resin may include an organic
group (e.g., an alkyl group or an aryl group) or a fluoro group as
a substituent. The organic group may include a fluoro group.
There is no particular limitation on the method for forming the
insulating layer 4021, and any of the following can be used
depending on a material thereof: a method such as a sputtering
method, an SOG method, spin coating, dipping, spray coating, or a
droplet discharging method (e.g., an ink-jet method, screen
printing, or offset printing); a tool such as doctor knife, roll
coater, curtain coater, or knife coater; or the like. The baking
step of the insulating layer 4021 also serves as the annealing step
of the semiconductor layer, whereby a semiconductor device can be
manufactured efficiently.
The pixel electrode layer 4030 and the counter electrode layer 4031
can be formed using a light-transmitting conductive material such
as indium oxide containing tungsten oxide, indium zinc oxide
containing tungsten oxide, indium oxide containing titanium oxide,
indium tin oxide containing titanium oxide, indium tin oxide
(hereinafter referred to as ITO), indium zinc oxide, or indium tin
oxide to which silicon oxide is added.
A conductive composition containing a conductive macromolecule
(also referred to as a conductive polymer) can be used for the
pixel electrode layer 4030 and the counter electrode layer 4031.
The pixel electrode formed using the conductive composition
preferably has a sheet resistance of 10000 ohms per square or less
and a transmittance of 70% or more at a wavelength of 550 nm.
Furthermore, the resistivity of the conductive macromolecule
contained in the conductive composition is preferably 0.1 .OMEGA.cm
or less.
As the conductive macromolecule, a so-called .pi.-electron
conjugated conductive polymer can be used. For example, it is
possible to use polyaniline or a derivative thereof, polypyrrole or
a derivative thereof, polythiophene or a derivative thereof, or a
copolymer of two or more kinds of them.
In addition, a variety of signals and potentials are supplied from
an FPC 4018 to the signal line driver circuit 4003 that is formed
separately, and the scan line driver circuit 4004 or the pixel
portion 4002.
A connection terminal electrode 4015 is formed from the same
conductive film as the pixel electrode layer 4030 included in the
liquid crystal element 4013, and a terminal electrode 4016 is
formed from the same conductive film as source and drain electrode
layers of the thin film transistors 4010 and 4011.
The connection terminal electrode 4015 is electrically connected to
a terminal included in the FPC 4018 through an anisotropic
conductive film 4019.
Note that FIGS. 10A1, 10A2 and 10B illustrate an example in which
the signal line driver circuit 4003 is formed separately and
mounted on the first substrate 4001; however, the present invention
is not limited to this structure. The scan line driver circuit may
be separately formed and then mounted, or only part of the signal
line driver circuit or part of the scan line driver circuit may be
separately formed and then mounted.
FIG. 20 illustrates an example of a liquid crystal display module
which is formed as a semiconductor device by using a TFT substrate
2600 manufactured in accordance with the manufacturing method
disclosed in this specification.
FIG. 20 illustrates an example of a liquid crystal display module,
in which the TFT substrate 2600 and a counter substrate 2601 are
bonded to each other with a sealant 2602, and a pixel portion 2603
including a TFT or the like, a display element 2604 including a
liquid crystal layer, and a coloring layer 2605 are provided
between the substrates to form a display region. The coloring layer
2605 is necessary to perform color display. In the case of the RGB
system, respective coloring layers corresponding to colors of red,
green, and blue are provided for respective pixels. Polarizing
plates 2606 and 2607 and a diffusion plate 2613 are provided
outside the TFT substrate 2600 and the counter substrate 2601. A
light source includes a cold cathode tube 2610 and a reflective
plate 2611. A circuit board 2612 is connected to a wiring circuit
portion 2608 of the TFT substrate 2600 through a flexible wiring
board 2609 and includes an external circuit such as a control
circuit or a power source circuit. The polarizing plate and the
liquid crystal layer may be stacked with a retardation plate
interposed therebetween.
For the liquid crystal display module, a twisted nematic (TN) mode,
an in-plane-switching (IPS) mode, a fringe field switching (FFS)
mode, a multi-domain vertical alignment (MVA) mode, a patterned
vertical alignment (PVA) mode, an axially symmetric aligned
micro-cell (ASM) mode, an optical compensated birefringence (OCB)
mode, a ferroelectric liquid crystal (FLC) mode, an
antiferroelectric liquid crystal (AFLC) mode, or the like can be
used.
Through the above process, a highly reliable liquid crystal display
panel as a semiconductor device can be manufactured.
This embodiment can be implemented in appropriate combination with
any of the structures described in the other embodiments.
Embodiment 7
An example of electronic paper will be described as a semiconductor
device.
The semiconductor device can be used for electronic paper that
drives electronic using an element electrically connected to a
switching element. The electronic paper is also referred to as an
electrophoretic display device (an electrophoretic display) and is
advantageous in that it has the same level of readability as plain
paper, it has lower power consumption than other display devices,
and it can be made thin and lightweight.
Electrophoretic displays can have various modes. Electrophoretic
displays contain a plurality of microcapsules dispersed in a
solvent or a solute, and each microcapsule contains first particles
which are positively charged and second particles which are
negatively charged. By application of an electric field to the
microcapsules, the particles in the microcapsules move in opposite
directions to each other and only the color of the particles
gathering on one side is displayed. Note that the first particles
and the second particles each contain pigment and do not move
without an electric field. Moreover, the first particles and the
second particles have different colors (which may be
colorless).
An electrophoretic display is thus a display that utilizes a
so-called dielectrophoretic effect by which a substance having a
high dielectric constant moves to a high-electric field region. An
electrophoretic display does not need to use a polarizing plate
which is required in a liquid crystal display device.
A solution in which the above microcapsules are dispersed in a
solvent is referred to as electronic ink. This electronic ink can
be printed on a surface of glass, plastic, cloth, paper, or the
like. Furthermore, by using a color filter or particles that have a
pigment, color display can also be achieved.
In addition, when a plurality of the microcapsules are arranged as
appropriate over an active matrix substrate so as to be interposed
between two electrodes, an active matrix display device can be
completed, and thus display can be performed by application of an
electric field to the microcapsules. For example, the active matrix
substrate obtained using the thin film transistor described in any
of Embodiments 1 to 4 can be used.
Note that the first particles and the second particles in the
microcapsules may each be formed using a single material selected
from a conductive material, an insulating material, a semiconductor
material, a magnetic material, a liquid crystal material, a
ferroelectric material, an electroluminescent material, an
electrochromic material, and a magnetophoretic material, or formed
using a composite material of any of these.
FIG. 9 illustrates active matrix electronic paper as an example of
the semiconductor device. A thin film transistor 581 used for the
semiconductor device can be manufactured in a manner similar to
that of the thin film transistor described in Embodiment 1 and is a
highly reliable thin film transistor including an oxide
semiconductor layer. The thin film transistor described in any of
Embodiments 2 to 4 can also be used as the thin film transistor 581
of this embodiment.
The electronic paper in FIG. 9 is an example of a display device
using a twisting ball display system. The twisting ball display
system refers to a method in which spherical particles each colored
in black and white are arranged between a first electrode layer and
a second electrode layer which are electrode layers used for a
display element, and a potential difference is generated between
the first electrode layer and the second electrode layer to control
orientation of the spherical particles, so that display is
performed.
The thin film transistor 581 sealed between a substrate 580 and a
substrate 596 is a thin film transistor having a bottom-gate
structure and is covered with an insulating film 583 that is in
contact with the semiconductor layer. A source or drain electrode
layer of the thin film transistor 581 is in contact with a first
electrode layer 587 through an opening formed in the insulating
film 583 and an insulating layer 585, whereby the thin film
transistor 581 is electrically connected to the first electrode
layer 587. Between the first electrode layer 587 and a second
electrode layer 588, spherical particles 589 each having a black
region 590a, a white region 590b, and a cavity 594 which is filled
with liquid around the black region 590a and the white region 590b
are provided. A space around the spherical particles 589 is filled
with a filler 595 such as a resin (see FIG. 9). The first electrode
layer 587 corresponds to a pixel electrode, and the second
electrode layer 588 corresponds to a common electrode. The second
electrode layer 588 is electrically connected to a common potential
line provided over the same substrate 580 as the thin film
transistor 581. With the use of a common connection portion, the
second electrode layer 588 can be electrically connected to the
common potential line through conductive particles provided between
the substrate 580 and the substrate 596.
Instead of the twisting ball, an electrophoretic element can also
be used. A microcapsule having a diameter of approximately 10 .mu.m
to 200 .mu.m in which transparent liquid, positively-charged white
microparticles, and negatively-charged black microparticles are
encapsulated is used. In the microcapsule which is provided between
the first electrode layer and the second electrode layer, when an
electric field is applied by the first electrode layer and the
second electrode layer, the white microparticles and the black
microparticles move to opposite sides from each other, so that
white or black can be displayed. A display element using this
principle is an electrophoretic display element and is generally
called electronic paper. The electrophoretic display element has
higher reflectance than a liquid crystal display element, and thus
an auxiliary light is unnecessary, power consumption is low, and a
display portion can be recognized even in a dim place. In addition,
even when power is not supplied to the display portion, an image
which has been displayed once can be maintained. Accordingly, a
displayed image can be stored even if a semiconductor device having
a display function (which may be referred to simply as a display
device or a semiconductor device provided with a display device) is
distanced from an electric wave source.
Through above process, highly reliable electronic paper as a
semiconductor device can be manufactured.
This embodiment can be implemented in appropriate combination with
any of the structures described in the other embodiments.
Embodiment 8
An example of a light-emitting display device will be described as
the semiconductor device. As a display element included in the
display device, a light-emitting element utilizing
electroluminescence is described in this embodiment. Light-emitting
elements utilizing electroluminescence are classified according to
whether a light-emitting material is an organic compound or an
inorganic compound. In general, the former is referred to as an
organic EL element, and the latter is referred to as an inorganic
EL element.
In an organic EL element, by application of voltage to a
light-emitting element, electrons and holes are separately injected
from a pair of electrodes into a layer containing a light-emitting
organic compound, and current flows. Then, the carriers (electrons
and holes) recombine, so that the light-emitting organic compound
is excited. The light-emitting organic compound returns to a ground
state from the excited state, thereby emitting light. Owing to such
a mechanism, this light-emitting element is referred to as a
current-excitation light-emitting element.
The inorganic EL elements are classified according to their element
structures into a dispersion-type inorganic EL element and a
thin-film inorganic EL element. A dispersion-type inorganic EL
element has a light-emitting layer where particles of a
light-emitting material are dispersed in a binder, and its light
emission mechanism is donor-acceptor recombination type light
emission which utilizes a donor level and an acceptor level. A
thin-film inorganic EL element has a structure where a
light-emitting layer is sandwiched between dielectric layers, which
are further sandwiched between electrodes, and its light emission
mechanism is localized type light emission that utilizes
inner-shell electron transition of metal ions. Note that
description is made in this embodiment using an organic EL element
as a light-emitting element.
FIG. 12 illustrates an example of a pixel configuration to which
digital time grayscale driving can be applied as an example of the
semiconductor device.
The configuration and operation of a pixel to which digital time
grayscale driving can be applied will be described. An example is
described in this embodiment in which one pixel includes two
n-channel transistors using an oxide semiconductor layer in a
channel formation region.
A pixel 6400 includes a switching transistor 6401, a driving
transistor 6402, a light-emitting element 6404, and a capacitor
6403. In the switching transistor 6401, a gate thereof is connected
to a scan line 6406, a first electrode thereof (one of source and
drain electrodes) is connected to a signal line 6405, and a second
electrode thereof (the other of the source and drain electrodes) is
connected to a gate of the driving transistor 6402. In the driving
transistor 6402, the gate thereof is connected to a power supply
line 6407 through the capacitor 6403, a first electrode thereof is
connected to the power supply line 6407, and a second electrode
thereof is connected to a first electrode (pixel electrode) of the
light-emitting element 6404. A second electrode of the
light-emitting element 6404 corresponds to a common electrode 6408.
The common electrode 6408 is electrically connected to a common
potential line provided over the same substrate.
Note that the second electrode (common electrode 6408) of the
light-emitting element 6404 is set to a low power supply potential.
Note that the low power supply potential is a potential satisfying
the low power supply potential <a high power supply potential
with reference to the high power supply potential that is set on
the power supply line 6407. As the low power supply potential, GND,
0 V, or the like may be employed, for example. The difference
between the high power supply potential and the low power supply
potential is applied to the light-emitting element 6404 so that
current flows through the light-emitting element 6404, whereby the
light-emitting element 6404 emits light. Thus, each potential is
set so that the difference between the high power supply potential
and the low power supply potential is greater than or equal to a
forward threshold voltage of the light-emitting element 6404.
When the gate capacitance of the driving transistor 6402 is used as
a substitute for the capacitor 6403, the capacitor 6403 can be
omitted. The gate capacitance of the driving transistor 6402 may be
formed between the channel region and the gate electrode.
In the case of using a voltage-input voltage driving method, a
video signal is inputted to the gate of the driving transistor 6402
so that the driving transistor 6402 is in either of two states of
being sufficiently turned on and turned off. That is, the driving
transistor 6402 operates in a linear region, and thus a voltage
higher than the voltage of the power supply line 6407 is applied to
the gate of the driving transistor 6402. Note that a voltage higher
than or equal to the following is applied to the signal line 6405:
power supply line voltage+V.sub.th of the driving transistor
6402.
In the case of performing analog grayscale driving instead of
digital time grayscale driving, the same pixel configuration as
FIG. 12 can be employed by inputting signals in a different
way.
In the case of performing analog grayscale driving, voltage higher
than or equal to the following is applied to the gate of the
driving transistor 6402: forward voltage of the light-emitting
element 6404+V.sub.th of the driving transistor 6402. The forward
voltage of the light-emitting element 6404 refers to voltage to
obtain a desired luminance, and includes at least forward threshold
voltage. By input of a video signal which enables the driving
transistor 6402 to operate in a saturation region, it is possible
to feed current to the light-emitting element 6404. In order that
the driving transistor 6402 can operate in the saturation region,
the potential of the power supply line 6407 is set higher than a
gate potential of the driving transistor 6402. When an analog video
signal is used, it is possible to feed current to the
light-emitting element 6404 in accordance with the video signal and
perform analog grayscale driving.
Note that the pixel configuration is not limited to that
illustrated in FIG. 12. For example, the pixel illustrated in FIG.
12 may further include a switch, a resistor, a capacitor, a
transistor, a logic circuit, or the like.
Next, structures of the light-emitting element will be described
with reference to FIGS. 13A to 13C. A cross-sectional structure of
a pixel will be described by taking an n-channel driving TFT as an
example. Driving TFTs 7001, 7011, and 7021 used for semiconductor
devices illustrated in FIGS. 13A, 13B, and 13C, respectively, can
be manufactured in a manner similar to that of the thin film
transistor described in Embodiment 1 and are highly reliable thin
film transistors each including an oxide semiconductor layer.
Alternatively, the thin film transistors described in any of
Embodiment 2 to 4 can be employed as the driving TFTs 7001, 7011,
and 7021.
In order to extract light emitted from the light-emitting element,
at least one of the anode and the cathode is required to transmit
light. A thin film transistor and a light-emitting element are
formed over a substrate. A light-emitting element can have a top
emission structure in which light is extracted through the surface
opposite to the substrate, a bottom emission structure in which
light is extracted through the surface on the substrate side, or a
dual emission structure in which light is extracted through the
surface opposite to the substrate and the surface on the substrate
side. The pixel configuration can be applied to a light-emitting
element having any of these emission structures.
A light-emitting element having a top emission structure will be
described with reference to FIG. 13A.
FIG. 13A is a cross-sectional view of a pixel in the case where the
driving TFT 7001 is an n-channel TFT and light is emitted from a
light-emitting element 7002 to an anode 7005 side. In FIG. 13A, a
cathode 7003 of the light-emitting element 7002 is electrically
connected to the driving TFT 7001, and a light-emitting layer 7004
and the anode 7005 are stacked in this order over the cathode 7003.
The cathode 7003 can be formed using a variety of conductive
materials as long as they have a low work function and reflect
light. For example, Ca, Al, MgAg, AlLi, or the like is preferably
used. The light-emitting layer 7004 may be formed as a single layer
or a plurality of layers stacked. When the light-emitting layer
7004 is formed as a plurality of layers, the light-emitting layer
7004 is formed by stacking an electron-injection layer, an
electron-transport layer, a light-emitting layer, a hole-transport
layer, and a hole-injection layer in this order over the cathode
7003. Note that not all of these layers need to be provided. The
anode 7005 may be formed using a light-transmitting conductive
material such as indium oxide including tungsten oxide, indium zinc
oxide including tungsten oxide, indium oxide including titanium
oxide, indium tin oxide including titanium oxide, indium tin oxide
(hereinafter referred to as ITO), indium zinc oxide, or indium tin
oxide to which silicon oxide is added.
The light-emitting element 7002 corresponds to a region where the
light-emitting layer 7004 is sandwiched between the cathode 7003
and the anode 7005. In the case of the pixel illustrated in FIG.
13A, light is emitted from the light-emitting element 7002 to the
anode 7005 side as indicated by an arrow.
Next, a light-emitting element having a bottom emission structure
will be described with reference to FIG. 13B. FIG. 13B is a
cross-sectional view of a pixel in the case where the driving TFT
7011 is an n-channel TFT and light is emitted from a light-emitting
element 7012 to a cathode 7013 side. In FIG. 13B, the cathode 7013
of the light-emitting element 7012 is formed over a
light-transmitting conductive film 7017 which is electrically
connected to the driving TFT 7011, and a light-emitting layer 7014
and an anode 7015 are stacked in this order over the cathode 7013.
Note that a light-blocking film 7016 for reflecting or blocking
light may be formed so as to cover the anode 7015 when the anode
7015 has a light-transmitting property. As in the case of FIG. 13A,
the cathode 7013 can be formed using a variety of conductive
materials as long as they have a low work function. Note that the
cathode 7013 is formed to have a thickness that can transmit light
(preferably, approximately 5 nm to 30 nm). For example, an aluminum
film with a thickness of 20 nm can be used as the cathode 7013. As
in the case of FIG. 13A, the light-emitting layer 7014 may be
formed using either a single layer or a plurality of layers
stacked. The anode 7015 is not required to transmit light, but can
be formed using a light-transmitting conductive material as in the
case of FIG. 13A. As the light-blocking film 7016, a metal which
reflects light can be used for example; however, the light-blocking
film 7016 is not limited to a metal film. For example, a resin to
which black pigments are added can also be used.
The light-emitting element 7012 corresponds to a region where the
light-emitting layer 7014 is sandwiched between the cathode 7013
and the anode 7015. In the case of the pixel illustrated in FIG.
13B, light is emitted from the light-emitting element 7012 to the
cathode 7013 side as indicated by an arrow.
Next, a light-emitting element having a dual emission structure
will be described with reference to FIG. 13C. In FIG. 13C, a
cathode 7023 of a light-emitting element 7022 is formed over a
light-transmitting conductive film 7027 which is electrically
connected to the driving TFT 7021, and a light-emitting layer 7024
and an anode 7025 are stacked in this order over the cathode 7023.
As in the case of FIG. 13A, the cathode 7023 can be formed using a
variety of conductive materials as long as they have a low work
function. Note that the cathode 7023 is formed to have a thickness
that can transmit light. For example, an aluminum film with a
thickness of 20 nm can be used as the cathode 7023. As in FIG. 13A,
the light-emitting layer 7024 may be formed using either a single
layer or a plurality of layers stacked. The anode 7025 can be
formed using a light-transmitting conductive material as in the
case of FIG. 13A.
The light-emitting element 7022 corresponds to a region where the
cathode 7023, the light-emitting layer 7024, and the anode 7025
overlap with one another. In the case of the pixel illustrated in
FIG. 13C, light is emitted from the light-emitting element 7022 to
both the anode 7025 side and the cathode 7023 side as indicated by
arrows.
Although an organic EL element is described in this embodiment as a
light-emitting element, an inorganic EL element can also be
provided as a light-emitting element.
Note that although the example is described in which a thin film
transistor (a driving TFT) which controls the driving of a
light-emitting element is electrically connected to the
light-emitting element, a structure may be employed in which a TFT
for current control is connected between the driving TFT and the
light-emitting element.
Note that the structure of the semiconductor device described in
this embodiment is not limited to those illustrated in FIGS. 13A to
13C and can be modified in various ways based on the spirit of
techniques disclosed in this specification.
Next, the appearance and cross section of a light-emitting display
panel (also referred to as a light-emitting panel), which is one
embodiment of the semiconductor device, will be described with
reference to FIGS. 11A and 11B. FIG. 11A is a plan view of a panel
in which a thin film transistor and a light-emitting element formed
over a first substrate are sealed between the first substrate and a
second substrate with a sealant. FIG. 11B is a cross-sectional view
taken along line H-I of FIG. 11A.
A sealant 4505 is provided to surround a pixel portion 4502, signal
line driver circuits 4503a and 4503b, and scan line driver circuits
4504a and 4504b, which are provided over a first substrate 4501. In
addition, a second substrate 4506 is provided over the pixel
portion 4502, the signal line driver circuits 4503a and 4503b, and
the scan line driver circuits 4504a and 4504b. Accordingly, the
pixel portion 4502, the signal line driver circuits 4503a and
4503b, and the scan line driver circuits 4504a and 4504b are sealed
together with a filler 4507, by the first substrate 4501, the
sealant 4505, and the second substrate 4506. It is preferable that
packaging (sealing) be thus performed with a protective film (such
as a bonding film or an ultraviolet curable resin film) or a cover
material with high air-tightness and little degasification so that
the display device is not thus exposed to the outside air.
The pixel portion 4502, the signal line driver circuits 4503a and
4503b, and the scan line driver circuits 4504a and 4504b formed
over the first substrate 4501 each include a plurality of thin film
transistors, and a thin film transistor 4510 included in the pixel
portion 4502 and a thin film transistor 4509 included in the signal
line driver circuit 4503a are illustrated as an example in FIG.
11B.
As the thin film transistors 4509 and 4510, the highly reliable
thin film transistor including an oxide semiconductor layer, which
is described in Embodiment 3, can be employed. Alternatively, the
thin film transistor described in any of Embodiments 1 to 4 can be
employed. In this embodiment, the thin film transistors 4509 and
4510 are n-channel thin film transistors.
Moreover, reference numeral 4511 denotes a light-emitting element.
A first electrode layer 4517 that is a pixel electrode included in
the light-emitting element 4511 is electrically connected to a
source or drain electrode layer of the thin film transistor 4510.
Note that a structure of the light-emitting element 4511 is not
limited to the stacked-layer structure described in this
embodiment, which includes the first electrode layer 4517, an
electroluminescent layer 4512, and a second electrode layer 4513.
The structure of the light-emitting element 4511 can be changed as
appropriate in a manner that depends on the direction in which
light is extracted from the light-emitting element 4511, for
example.
A partition wall 4520 is formed using an organic resin film, an
inorganic insulating film, or organic polysiloxane. It is
particularly preferable that the partition wall 4520 be formed
using a photosensitive material to have an opening over the first
electrode layer 4517 so that a sidewall of the opening is formed as
an inclined surface with continuous curvature.
The electroluminescent layer 4512 may be formed as a single layer
or a plurality of layers stacked.
A protective film may be formed over the second electrode layer
4513 and the partition wall 4520 in order to prevent oxygen,
hydrogen, moisture, carbon dioxide, or the like from entering the
light-emitting element 4511. As the protective film, a silicon
nitride film, a silicon nitride oxide film, a DLC film, or the like
can be formed.
A variety of signals and potentials are supplied from FPCs 4518a
and 4518b to the signal line driver circuits 4503a and 4503b, the
scan line driver circuits 4504a and 4504b, or the pixel portion
4502.
A connection terminal electrode 4515 is formed from the same
conductive film as the first electrode layer 4517 included in the
light-emitting element 4511, and a terminal electrode 4516 is
formed from the same conductive film as source and drain electrode
layers included in the thin film transistors 4509 and 4510.
The connection terminal electrode 4515 is electrically connected to
a terminal of the FPC 4518a through an anisotropic conductive film
4519.
The second substrate 4506 located in the direction in which light
is extracted from the light-emitting element 4511 needs to have a
light-transmitting property. In that case, a light-transmitting
material such as a glass plate, a plastic plate, a polyester film,
or an acrylic film is used.
As the filler 4507, an ultraviolet curable resin or a thermosetting
resin can be used, in addition to an inert gas such as nitrogen or
argon. For example, polyvinyl chloride (PVC), acrylic, polyimide,
an epoxy resin, a silicone resin, polyvinyl butyral (PVB), or
ethylene vinyl acetate (EVA) can be used. For example, nitrogen may
be used for the filler.
If needed, an optical film such as a polarizing plate, a circularly
polarizing plate (including an elliptically polarizing plate), a
retardation plate (a quarter-wave plate or a half-wave plate), or a
color filter may be provided as appropriate on a light-emitting
surface of the light-emitting element. Furthermore, the polarizing
plate or the circularly polarizing plate may be provided with an
anti-reflection film. For example, anti-glare treatment by which
reflected light can be diffused by projections and depressions on
the surface so that the glare is reduced can be performed.
The signal line driver circuits 4503a and 4503b and the scan line
driver circuits 4504a and 4504b may be mounted as driver circuits
formed using a single crystal semiconductor film or a
polycrystalline semiconductor film over a substrate separately
prepared. Alternatively, only the signal line driver circuits or
part thereof, or only the scan line driver circuits or part thereof
may be separately formed and mounted. The present invention is not
limited to the structure illustrated in FIGS. 11A and 11B.
Through the above process, a highly reliable light-emitting display
panel (light-emitting panel) as a semiconductor device can be
manufactured.
This embodiment can be implemented in appropriate combination with
any of the structures described in the other embodiments.
Embodiment 9
A semiconductor device disclosed in this specification can be
applied to electronic paper. Electronic paper can be used for
electronic appliances of a variety of fields as long as they can
display data. For example, electronic paper can be applied to an
electronic book (electronic book) reader, a poster, an
advertisement in a vehicle such as a train, or displays of various
cards such as a credit card. An example of the electronic
appliances is illustrated in FIG. 22.
FIG. 22 illustrates an example of an electronic book reader 2700.
For example, the electronic book reader 2700 includes two housings,
a housing 2701 and a housing 2703. The housing 2701 and the housing
2703 are combined with a hinge 2711 so that the electronic book
reader 2700 can be opened and closed with the hinge 2711 as an
axis. With such a structure, the electronic book reader 2700 can
operate like a paper book.
A display portion 2705 and a display portion 2707 are incorporated
in the housing 2701 and the housing 2703, respectively. The display
portion 2705 and the display portion 2707 may display one image or
different images. In the case where the display portion 2705 and
the display portion 2707 display different images, for example,
text can be displayed on a display portion on the right side (the
display portion 2705 in FIG. 22) and images can be displayed on a
display portion on the left side (the display portion 2707 in FIG.
22).
FIG. 22 illustrates an example in which the housing 2701 is
provided with an operation portion and the like. For example, the
housing 2701 is provided with a power switch 2721, an operation key
2723, a speaker 2725, and the like. With the operation key 2723,
pages can be turned. Note that a keyboard, a pointing device, and
the like may be provided on the same surface as the display portion
of the housing. Furthermore, an external connection terminal (an
earphone terminal, a USB terminal, a terminal that can be connected
to various cables such as an AC adapter and a USB cable, or the
like), a recording medium insertion portion, or the like may be
provided on the back surface or the side surface of the housing.
Moreover, the electronic book reader 2700 may have a function of an
electronic dictionary.
Further, the electronic book reader 2700 may send and receive
information wirelessly. Through wireless communication, desired
book data or the like can be purchased and downloaded from an
electronic book server.
Embodiment 10
A semiconductor device disclosed in this specification can be
applied to a variety of electronic appliances (including amusement
machines). Examples of electronic appliances include television
sets (also referred to as televisions or television receivers),
monitors of computers or the like, cameras such as digital cameras
or digital video cameras, digital photo frames, cellular phones
(also referred to as mobile phones or mobile phone sets), portable
game consoles, portable information terminals, audio reproducing
devices, large-sized game machines such as pachinko machines, and
the like.
FIG. 23A illustrates an example of a television set 9600. In the
television set 9600, a display portion 9603 is incorporated in a
housing 9601. Images can be displayed on the display portion 9603.
Here, the housing 9601 is supported by a stand 9605.
The television set 9600 can be operated with an operation switch of
the housing 9601 or a separate remote controller 9610. Channels and
volume can be controlled with an operation key 9609 of the remote
controller 9610 so that an image displayed on the display portion
9603 can be controlled. Furthermore, the remote controller 9610 may
be provided with a display portion 9607 which display data
outputted from the remote controller 9610.
Note that the television set 9600 is provided with a receiver, a
modem, and the like. With the receiver, a general television
broadcast can be received. Furthermore, when the television set
9600 is connected to a communication network by wired or wireless
connection via the modem, one-way (from a transmitter to a
receiver) or two-way (between a transmitter and a receiver, between
receivers, or the like) data communication can be performed.
FIG. 23B illustrates an example of a digital photo frame 9700. For
example, in the digital photo frame 9700, a display portion 9703 is
incorporated in a housing 9701. Various images can be displayed on
the display portion 9703. For example, the display portion 9703 can
display image data taken with a digital camera or the like to
function as a normal photo frame.
Note that the digital photo frame 9700 is provided with an
operation portion, an external connection portion (a USB terminal,
a terminal that can be connected to various cables such as a USB
cable, or the like), a recording medium insertion portion, and the
like. Although they may be provided on the same surface as the
display portion 9703, it is preferable to provide them on the side
surface or the back surface because the design thereof is improved.
For example, a memory in which image data taken with a digital
camera is stored is inserted in the recording medium insertion
portion of the digital photo frame 9700, whereby the image data can
be displayed on the display portion 9703.
The digital photo frame 9700 may send and receive information
wirelessly. Through wireless communication, desired image data can
be downloaded to be displayed.
FIG. 24A illustrates a portable amusement machine including two
housings, a housing 9881 and a housing 9891. The housings 9881 and
9891 are connected with a connection portion 9893 so as to be
opened and closed. A display portion 9882 and a display portion
9883 are incorporated in the housing 9881 and the housing 9891,
respectively. In addition, the portable amusement machine
illustrated in FIG. 24A includes a speaker portion 9884, a
recording medium insertion portion 9886, an LED lamp 9890, an input
unit (an operation key 9885, a connection terminal 9887, a sensor
9888 (a sensor having a function of measuring force, displacement,
position, speed, acceleration, angular velocity, rotational
frequency, distance, light, liquid, magnetism, temperature,
chemical substance, sound, time, hardness, electric field, current,
voltage, electric power, radiation, flow rate, humidity, gradient,
oscillation, odor, or infrared rays), and a microphone 9889), and
the like. It is needless to say that the structure of the portable
amusement machine is not limited to the above, and other structures
provided with at least a semiconductor device disclosed in this
specification may be employed. The portable amusement machine may
include other accessory equipment as appropriate. The portable
amusement machine illustrated in FIG. 24A has a function of reading
a program or data stored in a recording medium to display it on the
display portion, and a function of sharing information with another
portable amusement machine by wireless communication. The portable
amusement machine illustrated in FIG. 24A can have various
functions without limitation to the above.
FIG. 24B illustrates an example of a slot machine 9900 which is a
large-sized amusement machine. In the slot machine 9900, a display
portion 9903 is incorporated in a housing 9901. In addition, the
slot machine 9900 includes an operation unit such as a start lever
or a stop switch, a coin slot, a speaker, and the like. It is
needless to say that the structure of the slot machine 9900 is not
limited to the above, and other structures provided with at least a
semiconductor device disclosed in this specification may be
employed. The slot machine 9900 may include other accessory
equipment as appropriate.
FIG. 25A is a perspective view illustrating an example of a
portable computer.
In the portable computer in FIG. 25A, a top housing 9301 having a
display portion 9303 and a bottom housing 9302 having a keyboard
9304 can overlap with each other by closing a hinge unit which
connects the top housing 9301 and the bottom housing 9302. The
portable computer in FIG. 25A can be convenient for carrying, and
in the case of using the keyboard for input, the hinge unit is
opened so that the user can input looking at the display portion
9303.
The bottom housing 9302 includes a pointing device 9306 with which
input can be performed, in addition to the keyboard 9304. Further,
when the display portion 9303 is a touch input panel, input can be
performed by touching part of the display portion. The bottom
housing 9302 includes an arithmetic function portion such as a CPU
or hard disk. In addition, the bottom housing 9302 includes another
device, for example, an external connection port 9305 into which a
communication cable conformable to communication standards of a USB
is inserted.
The top housing 9301 further includes a display portion 9307 which
can be stored in the top housing 9301 by being slid therein. Thus,
a large display screen can be realized. In addition, the user can
adjust the orientation of a screen of the storable display portion
9307. When the storable display portion 9307 is a touch input
panel, input can be performed by touching part of the storable
display portion.
The display portion 9303 or the storable display portion 9307 is
formed using an image display device of a liquid crystal display
panel, a light-emitting display panel such as an organic
light-emitting element or an inorganic light-emitting element, or
the like.
In addition, the portable computer in FIG. 25A, which can be
provided with a receiver and the like, can receive a television
broadcast to display an image on the display portion. While the
hinge unit which connects the top housing 9301 and the bottom
housing 9302 is kept closed, the whole screen of the display
portion 9307 is exposed by sliding the display portion 9307 out and
the angle of the screen is adjusted; thus, the user can watch a
television broadcast. In this case, the hinge unit is not opened
and display is not performed on the display portion 9303. In
addition, start up of only a circuit which displays the television
broadcast is performed. Therefore, power consumption can be
minimized, which is advantageous for the portable computer whose
battery capacity is limited.
FIG. 25B is a perspective view illustrating an example of a
cellular phone that the user can wear on the wrist like a
wristwatch.
This cellular phone includes a main body which includes a battery
and a communication device having at least a telephone function; a
band portion 9204 which enables the main body to be wore on the
wrist; an adjusting portion 9205 which adjusts the band portion
9204 to fit the wrist; a display portion 9201; a speaker 9207; and
a microphone 9208.
In addition, the main body includes an operation switch 9203. The
operation switch 9203 serves, for example, as a switch for starting
a program for the Internet when the switch is pushed, in addition
to serving as a switch for turning on a power source, a switch for
shifting a display, a switch for instructing to start taking
images, or the like, and can be used so as to correspond to each
function.
Input to this cellular phone is operated by touching the display
portion 9201 with a finger, an input pen, or the like, by operating
the operation switch 9203, or by inputting voice into the
microphone 9208. Note that displayed buttons 9202 which are
displayed on the display portion 9201 are illustrated in FIG. 25B.
Input can be performed by touching the displayed buttons 9202 with
a finger or the like.
Further, the main body includes a camera portion 9206 including an
image pick-up unit having a function of converting an image of an
object, which is formed through a camera lens, to an electronic
image signal. Note that the camera portion is not necessarily
provided.
The cellular phone illustrated in FIG. 25B, which can be provided
with a receiver of a television broadcast and the like, can display
an image on the display portion 9201 by receiving a television
broadcast. In addition, the cellular phone illustrated in FIG. 25B
may be provided with a storage device and the like such as a
memory, and thus can record a television broadcast in the memory.
The cellular phone illustrated in FIG. 25B may have a function of
collecting location information, such as the GPS.
The display portion 9201 is formed using an image display device of
a liquid crystal display panel, a light-emitting display panel such
as an organic light-emitting element or an inorganic light-emitting
element, or the like. The cellular phone illustrated in FIG. 25B is
compact and lightweight and thus has limited battery capacity.
Therefore, a panel which can be driven with low power consumption
is preferably used as a display device for the display portion
9201.
Note that FIG. 25B illustrates the electronic appliance which is
worn on the wrist; however, this embodiment is not limited thereto
as long as a portable shape is employed.
EXAMPLE 1
In this example, in an oxide semiconductor layer including a region
having high oxygen density and a region having low oxygen density,
the simulation result thereof in change of the oxygen density
before and after heat treatment will be described with reference to
FIG. 34 and FIG. 21. As software for the simulation, Materials
Explorer 5.0 manufactured by Fujitsu Limited was used.
FIG. 34 illustrates a model of an oxide semiconductor layer which
was used for the simulation. Here, a structure in which a layer 703
having low oxygen density and a layer 705 having high oxygen
density were stacked was employed for an oxide semiconductor layer
701.
For the layer 703 having low oxygen density, an amorphous structure
was employed in which the numbers of In atoms, Ga atoms, and Zn
atoms were each 15 and the number of O atoms was 54.
In addition, for the layer 705 having high oxygen density, an
amorphous structure was employed in which the numbers of In atoms,
Ga atoms, and Zn atoms were each 15 and the number of O atoms was
66.
Moreover, the density of the oxide semiconductor layer 701 was set
at 5.9 g/cm.sup.3.
Next, the classical molecular dynamics (MD) simulation was
performed on the oxide semiconductor layer 701 under conditions of
NVT ensemble and a temperature of 250.degree. C. The time step was
set at 0.2 fs, and the total simulation time was set at 200 ps. In
addition, Born-Mayer-Huggins potential was used for the potentials
of metal-oxygen bonding and oxygen-oxygen bonding. Moreover,
movement of atoms at an upper end portion and a lower end portion
of the oxide semiconductor layer 701 was fixed.
Then, the simulation result is shown in FIG. 21. In z-axis
coordinates, the range of 0 nm to 1.15 nm indicates the layer 703
having low oxygen density, and the range of 1.15 nm to 2.3 nm
indicates the layer 705 having high oxygen density. The
distribution of oxygen densities before the MD simulation is
indicated by a solid line 707, and the distribution of oxygen
densities after the MD simulation is indicated by a dashed line
709.
The solid line 707 shows that the oxide semiconductor layer 701 has
higher oxygen densities in a region raging from the interface
between the layer 703 having low oxygen density and the layer 705
having high oxygen density to the layer 705 having high oxygen
density. On the other hand, in the dashed line 709, it is found
that the oxygen densities are uniform in the layer 703 having low
oxygen density and the layer 705 having high oxygen density.
As described above, when there is non-uniformity in the density of
oxygen densities as in the stack of the layer 703 having low oxygen
density and the layer 705 having high oxygen density, it is found
that the oxygen diffuses from where an oxygen density is higher to
where an oxygen density is lower by heat treatment and thus the
oxygen densities become uniform.
That is, as described in Embodiment 1, since the oxygen density at
the interface between the first oxide semiconductor layer 432 and
the oxide insulating film 407 is increased by formation of the
oxide insulating film 407 over the first oxide semiconductor layer
432, the oxygen diffuses to the first oxide semiconductor layer 432
where the oxygen density is low and thus the first oxide
semiconductor layer 432 has higher resistance. As described above,
reliability of a thin film transistor can be improved.
The present application is based on Japanese Patent Application
serial No. 2009-156414 filed with Japan Patent Office on Jun. 30,
2009, the entire contents of which are hereby incorporated by
reference.
EXPLANATION OF REFERENCE
34: oxide semiconductor layer, 100: substrate, 101: gate electrode
layer, 102: gate insulating layer, 103: semiconductor layer, 107:
protective insulating layer, 108: capacitor wiring, 110: pixel
electrode layer, 121: terminal, 122: terminal, 125: contact hole,
126: contact hole, 127: contact hole, 128: transparent conductive
film, 129: transparent conductive film, 131: oxide semiconductor
film, 132: conductive film, 133: oxide semiconductor film, 134:
oxide semiconductor layer, 135: oxide semiconductor layer, 136:
oxide semiconductor film, 137: oxide semiconductor film, 138: oxide
semiconductor layer, 150: terminal, 151: terminal, 152: gate
insulating layer, 153: connection electrode layer, 154: protective
insulating film, 155: transparent conductive film, 156: electrode
layer, 170: thin film transistor, 400: substrate, 401: gate
electrode layer, 402: gate insulating layer, 403: semiconductor
layer, 407: oxide insulating film, 408: conductive layer, 409:
conductive layer, 410: insulating layer, 411: pixel electrode
layer, 430: oxide semiconductor film, 431: oxide semiconductor
layer, 432: oxide semiconductor layer, 433: oxide semiconductor
film, 434: oxide semiconductor film, 435: oxide semiconductor film,
436: oxide semiconductor layer, 470: thin film transistor, 471:
thin film transistor, 472: thin film transistor, 580: substrate,
581: thin film transistor, 583: insulating film, 585: insulating
layer, 587: electrode layer, 588: electrode layer, 589: spherical
particle, 594: cavity, 595: filler, 596: substrate, 601: electric
furnace, 602: chamber, 603: heater, 604: substrate, 605: susceptor,
606: gas supply unit, 607: evacuation unit, 703: layer having low
oxygen density, 705: layer having high oxygen density, 707: solid
line, 709: dashed line, 104a: source or drain region, 104b: source
or drain region, 105a: source or drain electrode layer, 105b:
source or drain electrode layer, 2600: TFT substrate, 2601: counter
substrate, 2602: sealant, 2603: pixel portion, 2604: display
element, 605: coloring layer, 2606: polarizing plate, 2607:
polarizing plate, 2608: wiring circuit portion, 2609: flexible
wiring board, 2610: cold cathode tube, 2611: reflective plate,
2612: circuit board, 2613: diffusion plate, 2700: electronic book
reader, 2701: housing, 2703: housing, 2705: display portion, 2707:
display portion, 2711: hinge, 2721: power switch, 2723: operation
key, 2725: speaker, 4001: substrate, 4002: pixel portion, 4003:
signal driver circuit, 4004: scan line driver circuit, 4005:
sealant, 4006: substrate, 4008: liquid crystal layer, 4010: thin
film transistor, 4011: thin film transistor, 4013: liquid crystal
element, 4015: connection terminal electrode, 4016: terminal
electrode, 4018: FPC, 4019: anisotropic conductive film, 4020:
insulating layer, 4021: insulating layer, 4030: pixel electrode
layer, 4031: counter electrode layer, 4032: insulating layer, 404a:
source or drain region, 404b: source or drain region, 405a: source
or drain electrode layer, 405b: source or drain electrode layer,
4501: substrate, 4502: pixel portion, 4505: sealant, 4506:
substrate, 4507: filler, 4509: thin film transistor, 4510: thin
film transistor, 4511: light-emitting element, 4512:
electroluminescent layer, 4513: second electrode layer, 4515:
connection terminal electrode, 4516: terminal electrode, 4517:
first electrode layer, 4519: anisotropic conductive film, 4520:
partition wall, 5300: substrate, 5301: pixel portion, 5302: scan
line driver circuit, 5303: signal line driver circuit, 5400:
substrate, 5401: pixel portion, 5402: scan line driver circuit,
5403: signal line driver circuit, 5404: scan line driver circuit,
5501: wiring, 5502: wiring, 5503: wiring, 5504: wiring, 5505:
wiring, 5506: wiring, 5543: node, 5544: node, 5571: thin film
transistor, 5572: thin film transistor, 5573: thin film transistor,
5574: thin film transistor, 5575: thin film transistor, 5576: thin
film transistor, 5577: thin film transistor, 5578: thin film
transistor, 5601: driver IC, 5602: switch group, 5611: wiring,
5612: wiring, 5613: wiring, 5621: wiring, 5701: flip-flop, 5711:
wiring, 5712: wiring, 5713: wiring, 5714: wiring, 5715: wiring,
5716: wiring, 5717: wiring, 5721: signal, 5821: signal, 590a: black
region, 590b: white region, 611a: gas supply source, 611b: a gas
supply source, 612a: pressure adjusting valve, 612b: pressure
adjusting valve, 613a: refiner, 613b: refiner, 614a: mass flow
controller, 614b: mass flow controller, 615a: stop valve, 615b:
stop valve, 6400: pixel, 6401: switching transistor, 6402: driving
transistor, 6403: capacitor element, 6404: light-emitting element,
6405: signal line, 6406: scan line, 6407: power supply line, 6408:
common electrode, 7001: TFT, 7002: light-emitting element, 7003:
cathode, 7004: light-emitting layer, 7005: anode, 7011: driving
TFT, 7012: light-emitting element, 7013: cathode, 7014:
light-emitting layer, 7015: anode, 7016: blocking film, 7017:
conductive film, 7021: driving TFT, 7022: light-emitting element,
7023: cathode, 7024: light-emitting layer, 7025: anode, 7027:
conductive film, 9201: display portion, 9202: displayed button,
9203: operation switch, 9204: band portion, 9205: adjusting
portion, 9206: camera portion, 9207: speaker, 9208: microphone,
9301: top housing, 9302: bottom housing, 9303: display portion,
9304: key board, 9305: external connection port, 9306: pointing
device, 9307: display portion, 9600: television set, 9601: housing,
9603: display portion, 9605: stand, 9607: display portion, 9609:
operation key, 9610: remote controller, 9700: digital photo frame,
9701: housing, 9703: display portion, 9881: housing, 9882: display
portion, 9883: display portion, 9884: speaker portion, 9885:
operation key, 9886: recording medium insertion portion, 9887:
connection terminal, 9888: sensor, 9889: microphone, 9890: LED
lamp, 9891: housing, 9893: connection portion, 9900: slot machine,
9901: housing, 9903: display portion, 4503a: signal line driver
circuit, 4503b: signal line driver circuit, 4504a: scan line driver
circuit, 4504b: scan line driver circuit, 4518a: FPC, 4518b: FPC,
5603a: thin film transistor, 5603b: thin film transistor, 5603c:
thin film transistor, 5703a: timing, 5703b: timing, 5703c: timing,
5803a: timing, 5803b: timing, 5803c: timing.
* * * * *